A system for offlining content automatically.

This specification is an experimental breakup of the HTML specification. You can see the full list on the index page and take part in the discussion in the repository.

Introduction

This section is non-normative.

In order to enable users to continue interacting with Web applications and documents even when their network connection is unavailable — for instance, because they are traveling outside of their ISP's coverage area — authors can provide a manifest which lists the files that are needed for the Web application to work offline and which causes the user's browser to keep a copy of the files for use offline.

To illustrate this, consider a simple clock applet consisting of an HTML page "clock.html", a CSS style sheet "clock.css", and a JavaScript script "clock.js".

Before adding the manifest, these three files might look like this:

<!-- clock.html -->
<!DOCTYPE HTML>
<html>
 <head>
  <title>Clock</title>
  <script src="clock.js"></script>
  <link rel="stylesheet" href="clock.css">
 </head>
 <body>
  <p>The time is: <output id="clock"></output></p>
 </body>
</html>
/* clock.css */
output { font: 2em sans-serif; }
/* clock.js */
setInterval(function () {
    document.getElementById('clock').value = new Date();
}, 1000);

If the user tries to open the "clock.html" page while offline, though, the user agent (unless it happens to have it still in the local cache) will fail with an error.

The author can instead provide a manifest of the three files, say "clock.appcache":

CACHE MANIFEST
clock.html
clock.css
clock.js

With a small change to the HTML file, the manifest (served as text/cache-manifest) is linked to the application:

<!-- clock.html -->
<!DOCTYPE HTML>
<html manifest="clock.appcache">
 <head>
  <title>Clock</title>
  <script src="clock.js"></script>
  <link rel="stylesheet" href="clock.css">
 </head>
 <body>
  <p>The time is: <output id="clock"></output></p>
 </body>
</html>

Now, if the user goes to the page, the browser will cache the files and make them available even when the user is offline.

Authors are encouraged to include the main page in the manifest also, but in practice the page that referenced the manifest is automatically cached even if it isn't explicitly mentioned.

With the exception of "no-store" directive, HTTP cache headers and restrictions on caching pages served over TLS (encrypted, using https:) are overridden by manifests. Thus, pages will not expire from an application cache before the user agent has updated it, and even applications served over TLS can be made to work offline.

View this example online.

Supporting offline caching for legacy applications

This section is non-normative.

The application cache feature works best if the application logic is separate from the application and user data, with the logic (markup, scripts, style sheets, images, etc) listed in the manifest and stored in the application cache, with a finite number of static HTML pages for the application, and with the application and user data stored in Web Storage or a client-side Indexed Database, updated dynamically using Web Sockets, XMLHttpRequest, server-sent events, or some other similar mechanism.

This model results in a fast experience for the user: the application immediately loads, and fresh data is obtained as fast as the network will allow it (possibly while stale data shows).

Legacy applications, however, tend to be designed so that the user data and the logic are mixed together in the HTML, with each operation resulting in a new HTML page from the server.

For example, consider a news application. The typical architecture of such an application, when not using the application cache feature, is that the user fetches the main page, and the server returns a dynamically-generated page with the current headlines and the user interface logic mixed together.

A news application designed for the application cache feature, however, would instead have the main page just consist of the logic, and would then have the main page fetch the data separately from the server, e.g. using XMLHttpRequest.

The mixed-content model does not work well with the application cache feature: since the content is cached, it would result in the user always seeing the stale data from the previous time the cache was updated.

While there is no way to make the legacy model work as fast as the separated model, it can at least be retrofitted for offline use using the prefer-online application cache mode. To do so, list all the static resources used by the HTML page you want to have work offline in an application cache manifest, use the manifest attribute to select that manifest from the HTML file, and then add the following line at the bottom of the manifest:

SETTINGS:
prefer-online
NETWORK:
*

This causes the application cache to only be used for master entries when the user is offline, and causes the application cache to be used as an atomic HTTP cache (essentially pinning resources listed in the manifest), while allowing all resources not listed in the manifest to be accessed normally when the user is online.

Event summary

This section is non-normative.

When the user visits a page that declares a manifest, the browser will try to update the cache. It does this by fetching a copy of the manifest and, if the manifest has changed since the user agent last saw it, redownloading all the resources it mentions and caching them anew.

As this is going on, a number of events get fired on the ApplicationCache object to keep the script updated as to the state of the cache update, so that the user can be notified appropriately. The events are as follows:

Event name Interface Fired when... Next events
checking Event The user agent is checking for an update, or attempting to download the manifest for the first time. This is always the first event in the sequence. noupdate, downloading, obsolete, error
noupdate Event The manifest hadn't changed. Last event in sequence.
downloading Event The user agent has found an update and is fetching it, or is downloading the resources listed by the manifest for the first time. progress, error, cached, updateready
progress ProgressEvent The user agent is downloading resources listed by the manifest. The event object's total attribute returns the total number of files to be downloaded. The event object's loaded attribute returns the number of files processed so far. progress, error, cached, updateready
cached Event The resources listed in the manifest have been downloaded, and the application is now cached. Last event in sequence.
updateready Event The resources listed in the manifest have been newly redownloaded, and the script can use swapCache() to switch to the new cache. Last event in sequence.
obsolete Event The manifest was found to have become a 404 or 410 page, so the application cache is being deleted. Last event in sequence.
error Event The manifest was a 404 or 410 page, so the attempt to cache the application has been aborted. Last event in sequence.
The manifest hadn't changed, but the page referencing the manifest failed to download properly.
A fatal error occurred while fetching the resources listed in the manifest.
The manifest changed while the update was being run. The user agent will try fetching the files again momentarily.

These events are cancelable; their default action is for the user agent to show download progress information. If the page shows its own update UI, canceling the events will prevent the user agent from showing redundant progress information.

Application caches

An application cache is a set of cached resources consisting of:

Each application cache has a completeness flag, which is either complete or incomplete.

An application cache group is a group of application caches, identified by the absolute URL of a resource manifest which is used to populate the caches in the group.

An application cache is newer than another if it was created after the other (in other words, application caches in an application cache group have a chronological order).

Only the newest application cache in an application cache group can have its completeness flag set to incomplete; the others are always all complete.

Each application cache group has an update status, which is one of the following: idle, checking, downloading.

A relevant application cache is an application cache that is the newest in its group to be complete.

Each application cache group has a list of pending master entries. Each entry in this list consists of a resource and a corresponding Document object. It is used during the application cache download process to ensure that new master entries are cached even if the application cache download process was already running for their application cache group when they were loaded.

An application cache group can be marked as obsolete, meaning that it must be ignored when looking at what application cache groups exist.

A cache host is a Document or a SharedWorkerGlobalScope object. A cache host can be associated with an application cache.

A Document initially is not associated with an application cache, but can become associated with one early during the page load process, when steps in the parser and in the navigation sections cause cache selection to occur.

A SharedWorkerGlobalScope can be associated with an application cache when it is created.

Each cache host has an associated ApplicationCache object.

Multiple application caches in different application cache groups can contain the same resource, e.g. if the manifests all reference that resource. If the user agent is to select an application cache from a list of relevant application caches that contain a resource, the user agent must use the application cache that the user most likely wants to see the resource from, taking into account the following:

A URL matches a fallback namespace if there exists a relevant application cache whose manifest's URL has the same origin as the URL in question, and that has a fallback namespace that is a prefix match for the URL being examined. If multiple fallback namespaces match the same URL, the longest one is the one that matches. A URL looking for a fallback namespace can match more than one application cache at a time, but only matches one namespace in each cache.

If a manifest http://example.com/app1/manifest declares that http://example.com/resources/images is a fallback namespace, and the user navigates to HTTP://EXAMPLE.COM:80/resources/images/cat.png, then the user agent will decide that the application cache identified by http://example.com/app1/manifest contains a namespace with a match for that URL.

The cache manifest syntax

Some sample manifests

This section is non-normative.

This example manifest requires two images and a style sheet to be cached and whitelists a CGI script.

CACHE MANIFEST
# the above line is required

# this is a comment
# there can be as many of these anywhere in the file
# they are all ignored
  # comments can have spaces before them
  # but must be alone on the line

# blank lines are ignored too

# these are files that need to be cached they can either be listed
# first, or a "CACHE:" header could be put before them, as is done
# lower down.
images/sound-icon.png
images/background.png
# note that each file has to be put on its own line

# here is a file for the online whitelist -- it isn't cached, and
# references to this file will bypass the cache, always hitting the
# network (or trying to, if the user is offline).
NETWORK:
comm.cgi

# here is another set of files to cache, this time just the CSS file.
CACHE:
style/default.css

It could equally well be written as follows:

CACHE MANIFEST
NETWORK:
comm.cgi
CACHE:
style/default.css
images/sound-icon.png
images/background.png

Offline application cache manifests can use absolute paths or even absolute URLs:

CACHE MANIFEST

/main/home
/main/app.js
/settings/home
/settings/app.js
http://img.example.com/logo.png
http://img.example.com/check.png
http://img.example.com/cross.png

The following manifest defines a catch-all error page that is displayed for any page on the site while the user is offline. It also specifies that the online whitelist wildcard flag is open, meaning that accesses to resources on other sites will not be blocked. (Resources on the same site are already not blocked because of the catch-all fallback namespace.)

So long as all pages on the site reference this manifest, they will get cached locally as they are fetched, so that subsequent hits to the same page will load the page immediately from the cache. Until the manifest is changed, those pages will not be fetched from the server again. When the manifest changes, then all the files will be redownloaded.

Subresources, such as style sheets, images, etc, would only be cached using the regular HTTP caching semantics, however.

CACHE MANIFEST
FALLBACK:
/ /offline.html
NETWORK:
*

Writing cache manifests

Manifests must be served using the text/cache-manifest MIME type. All resources served using the text/cache-manifest MIME type must follow the syntax of application cache manifests, as described in this section.

An application cache manifest is a text file, whose text is encoded using UTF-8. Data in application cache manifests is line-based. Newlines must be represented by U+000A LINE FEED (LF) characters, U+000D CARRIAGE RETURN (CR) characters, or U+000D CARRIAGE RETURN (CR) U+000A LINE FEED (LF) pairs. [[!ENCODING]]

This is a willful violation of RFC 2046, which requires all text/* types to only allow CRLF line breaks. This requirement, however, is outdated; the use of CR, LF, and CRLF line breaks is commonly supported and indeed sometimes CRLF is not supported by text editors. [[!RFC2046]]

The first line of an application cache manifest must consist of the string "CACHE", a single U+0020 SPACE character, the string "MANIFEST", and either a U+0020 SPACE character, a U+0009 CHARACTER TABULATION (tab) character, a U+000A LINE FEED (LF) character, or a U+000D CARRIAGE RETURN (CR) character. The first line may optionally be preceded by a U+FEFF BYTE ORDER MARK (BOM) character. If any other text is found on the first line, it is ignored.

Subsequent lines, if any, must all be one of the following:

A blank line

Blank lines must consist of zero or more U+0020 SPACE and U+0009 CHARACTER TABULATION (tab) characters only.

A comment

Comment lines must consist of zero or more U+0020 SPACE and U+0009 CHARACTER TABULATION (tab) characters, followed by a single U+0023 NUMBER SIGN character (#), followed by zero or more characters other than U+000A LINE FEED (LF) and U+000D CARRIAGE RETURN (CR) characters.

Comments must be on a line on their own. If they were to be included on a line with a URL, the "#" would be mistaken for part of a fragment identifier.

A section header

Section headers change the current section. There are four possible section headers:

CACHE:
Switches to the explicit section.
FALLBACK:
Switches to the fallback section.
NETWORK:
Switches to the online whitelist section.
SETTINGS:
Switches to the settings section.

Section header lines must consist of zero or more U+0020 SPACE and U+0009 CHARACTER TABULATION (tab) characters, followed by one of the names above (including the U+003A COLON character (:)) followed by zero or more U+0020 SPACE and U+0009 CHARACTER TABULATION (tab) characters.

Ironically, by default, the current section is the explicit section.

Data for the current section

The format that data lines must take depends on the current section.

When the current section is the explicit section, data lines must consist of zero or more U+0020 SPACE and U+0009 CHARACTER TABULATION (tab) characters, a valid URL identifying a resource other than the manifest itself, and then zero or more U+0020 SPACE and U+0009 CHARACTER TABULATION (tab) characters.

When the current section is the fallback section, data lines must consist of zero or more U+0020 SPACE and U+0009 CHARACTER TABULATION (tab) characters, a valid URL identifying a resource other than the manifest itself, one or more U+0020 SPACE and U+0009 CHARACTER TABULATION (tab) characters, another valid URL identifying a resource other than the manifest itself, and then zero or more U+0020 SPACE and U+0009 CHARACTER TABULATION (tab) characters.

When the current section is the online whitelist section, data lines must consist of zero or more U+0020 SPACE and U+0009 CHARACTER TABULATION (tab) characters, either a single U+002A ASTERISK character (*) or a valid URL identifying a resource other than the manifest itself, and then zero or more U+0020 SPACE and U+0009 CHARACTER TABULATION (tab) characters.

When the current section is the settings section, data lines must consist of zero or more U+0020 SPACE and U+0009 CHARACTER TABULATION (tab) characters, a setting, and then zero or more U+0020 SPACE and U+0009 CHARACTER TABULATION (tab) characters.

Currently only one setting is defined:

The cache mode setting
This consists of the string "prefer-online". It sets the cache mode to prefer-online. (The cache mode defaults to fast.)

Within a settings section, each setting must occur no more than once.

Manifests may contain sections more than once. Sections may be empty.

URLs that are to be fallback pages associated with fallback namespaces, and those namespaces themselves, must be given in fallback sections, with the namespace being the first URL of the data line, and the corresponding fallback page being the second URL. All the other pages to be cached must be listed in explicit sections.

Fallback namespaces and fallback entries must have the same origin as the manifest itself. Fallback namespaces must also be in the same path as the manifest's URL.

A fallback namespace must not be listed more than once.

Namespaces that the user agent is to put into the online whitelist must all be specified in online whitelist sections. (This is needed for any URL that the page is intending to use to communicate back to the server.) To specify that all URLs are automatically whitelisted in this way, a U+002A ASTERISK character (*) may be specified as one of the URLs.

Authors should not include namespaces in the online whitelist for which another namespace in the online whitelist is a prefix match.

Relative URLs must be given relative to the manifest's own URL. All URLs in the manifest must have the same scheme as the manifest itself (either explicitly or implicitly, through the use of relative URLs). [[!URL]]

URLs in manifests must not have fragment identifiers (i.e. the U+0023 NUMBER SIGN character isn't allowed in URLs in manifests).

Fallback namespaces and namespaces in the online whitelist are matched by prefix match.

Parsing cache manifests

When a user agent is to parse a manifest, it means that the user agent must run the following steps:

  1. UTF-8 decode the byte stream corresponding with the manifest to be parsed.

    The UTF-8 decode algorithm strips a leading BOM, if any.

  2. Let base URL be the absolute URL representing the manifest.

  3. Apply the URL parser to base URL, and let manifest path be the path component thus obtained.

  4. Remove all the characters in manifest path after the last U+002F SOLIDUS character (/), if any. (The first character and the last character in manifest path after this step will both be slashes, the URL path separator character.)

  5. Apply the URL parser steps to the base URL, so that the components from its parsed URL can be used by the subseqent steps of this algorithm.

  6. Let explicit URLs be an initially empty list of absolute URLs for explicit entries.

  7. Let fallback URLs be an initially empty mapping of fallback namespaces to absolute URLs for fallback entries.

  8. Let online whitelist namespaces be an initially empty list of absolute URLs for an online whitelist.

  9. Let online whitelist wildcard flag be blocking.

  10. Let cache mode flag be fast.

  11. Let input be the decoded text of the manifest's byte stream.

  12. Let position be a pointer into input, initially pointing at the first character.

  13. If the characters starting from position are "CACHE", followed by a U+0020 SPACE character, followed by "MANIFEST", then advance position to the next character after those. Otherwise, this isn't a cache manifest; abort this algorithm with a failure while checking for the magic signature.

  14. If the character at position is neither a U+0020 SPACE character, a U+0009 CHARACTER TABULATION (tab) character, U+000A LINE FEED (LF) character, nor a U+000D CARRIAGE RETURN (CR) character, then this isn't a cache manifest; abort this algorithm with a failure while checking for the magic signature.

  15. This is a cache manifest. The algorithm cannot fail beyond this point (though bogus lines can get ignored).

  16. Collect a sequence of characters that are not U+000A LINE FEED (LF) or U+000D CARRIAGE RETURN (CR) characters, and ignore those characters. (Extra text on the first line, after the signature, is ignored.)

  17. Let mode be "explicit".

  18. Start of line: If position is past the end of input, then jump to the last step. Otherwise, collect a sequence of characters that are U+000A LINE FEED (LF), U+000D CARRIAGE RETURN (CR), U+0020 SPACE, or U+0009 CHARACTER TABULATION (tab) characters.

  19. Now, collect a sequence of characters that are not U+000A LINE FEED (LF) or U+000D CARRIAGE RETURN (CR) characters, and let the result be line.

  20. Drop any trailing U+0020 SPACE and U+0009 CHARACTER TABULATION (tab) characters at the end of line.

  21. If line is the empty string, then jump back to the step labeled start of line.

  22. If the first character in line is a U+0023 NUMBER SIGN character (#), then jump back to the step labeled start of line.

  23. If line equals "CACHE:" (the word "CACHE" followed by a U+003A COLON character (:)), then set mode to "explicit" and jump back to the step labeled start of line.

  24. If line equals "FALLBACK:" (the word "FALLBACK" followed by a U+003A COLON character (:)), then set mode to "fallback" and jump back to the step labeled start of line.

  25. If line equals "NETWORK:" (the word "NETWORK" followed by a U+003A COLON character (:)), then set mode to "online whitelist" and jump back to the step labeled start of line.

  26. If line equals "SETTINGS:" (the word "SETTINGS" followed by a U+003A COLON character (:)), then set mode to "settings" and jump back to the step labeled start of line.

  27. If line ends with a U+003A COLON character (:), then set mode to "unknown" and jump back to the step labeled start of line.

  28. This is either a data line or it is syntactically incorrect.

  29. Let position be a pointer into line, initially pointing at the start of the string.

  30. Let tokens be a list of strings, initially empty.

  31. While position doesn't point past the end of line:

    1. Let current token be an empty string.

    2. While position doesn't point past the end of line and the character at position is neither a U+0020 SPACE nor a U+0009 CHARACTER TABULATION (tab) character, add the character at position to current token and advance position to the next character in input.

    3. Add current token to the tokens list.

    4. While position doesn't point past the end of line and the character at position is either a U+0020 SPACE or a U+0009 CHARACTER TABULATION (tab) character, advance position to the next character in input.

  32. Process tokens as follows:

    If mode is "explicit"

    Resolve the first item in tokens, relative to base URL, with the URL character encoding set to UTF-8; ignore the rest.

    If this fails, then jump back to the step labeled start of line.

    If the resulting parsed URL has a different scheme component than base URL (the manifest's URL), then jump back to the step labeled start of line.

    Let new URL be the result of applying the URL serialiser algorithm to the resulting parsed URL, with the exclude fragment flag set.

    Add new URL to the explicit URLs.

    If mode is "fallback"

    Let part one be the first token in tokens, and let part two be the second token in tokens.

    Resolve part one and part two, relative to base URL, with the URL character encoding set to UTF-8.

    If either fails, then jump back to the step labeled start of line.

    If the absolute URL corresponding to either part one or part two does not have the same origin as the manifest's URL, then jump back to the step labeled start of line.

    Let part one path be the path component of the resulting parsed URL for part one.

    If manifest path is not a prefix match for part one path, then jump back to the step labeled start of line.

    Let part one be the result of applying the URL serialiser algorithm to the first resulting parsed URL, with the exclude fragment flag set.

    Let part two be the result of applying the URL serialiser algorithm to the second resulting parsed URL, with the exclude fragment flag set.

    If part one is already in the fallback URLs mapping as a fallback namespace, then jump back to the step labeled start of line.

    Otherwise, add part one to the fallback URLs mapping as a fallback namespace, mapped to part two as the fallback entry.

    If mode is "online whitelist"

    If the first item in tokens is a U+002A ASTERISK character (*), then set online whitelist wildcard flag to open and jump back to the step labeled start of line.

    Otherwise, resolve the first item in tokens, relative to base URL, with the URL character encoding set to UTF-8; ignore the rest.

    If this fails, then jump back to the step labeled start of line.

    If the resulting parsed URL has a different scheme component than base URL (the manifest's URL), then jump back to the step labeled start of line.

    Let new URL be the result of applying the URL serialiser algorithm to the resulting parsed URL, with the exclude fragment flag set.

    Add new URL to the online whitelist namespaces.

    If mode is "settings"

    If tokens contains a single token, and that token is a case-sensitive match for the string "prefer-online", then set cache mode flag to prefer-online and jump back to the step labeled start of line.

    Otherwise, the line is an unsupported setting: do nothing; the line is ignored.

    If mode is "unknown"

    Do nothing. The line is ignored.

  33. Jump back to the step labeled start of line. (That step jumps to the next, and last, step when the end of the file is reached.)

  34. Return the explicit URLs list, the fallback URLs mapping, the online whitelist namespaces, the online whitelist wildcard flag, and the cache mode flag.

The resource that declares the manifest (with the manifest attribute) will always get taken from the cache, whether it is listed in the cache or not, even if it is listed in an online whitelist namespace.

If a resource is listed in the explicit section or as a fallback entry in the fallback section, the resource will always be taken from the cache, regardless of any other matching entries in the fallback namespaces or online whitelist namespaces.

When a fallback namespace and an online whitelist namespace overlap, the online whitelist namespace has priority.

The online whitelist wildcard flag is applied last, only for URLs that match neither the online whitelist namespace nor the fallback namespace and that are not listed in the explicit section.

Downloading or updating an application cache

When the user agent is required (by other parts of this specification) to start the application cache download process for an absolute URL purported to identify a manifest, or for an application cache group, potentially given a particular cache host, and potentially given a master resource, the user agent must run the steps below. These steps are always run in parallel with the event loop tasks.

Some of these steps have requirements that only apply if the user agent shows caching progress. Support for this is optional. Caching progress UI could consist of a progress bar or message panel in the user agent's interface, or an overlay, or something else. Certain events fired during the application cache download process allow the script to override the display of such an interface. (Such events are delayed until after the load event has fired.) The goal of this is to allow Web applications to provide more seamless update mechanisms, hiding from the user the mechanics of the application cache mechanism. User agents may display user interfaces independent of this, but are encouraged to not show prominent update progress notifications for applications that cancel the relevant events.

The application cache download process steps are as follows:

  1. Optionally, wait until the permission to start the application cache download process has been obtained from the user and until the user agent is confident that the network is available. This could include doing nothing until the user explicitly opts-in to caching the site, or could involve prompting the user for permission. The algorithm might never get past this point. (This step is particularly intended to be used by user agents running on severely space-constrained devices or in highly privacy-sensitive environments).

  2. Atomically, so as to avoid race conditions, perform the following substeps:

    1. Pick the appropriate substeps:

      If these steps were invoked with an absolute URL purported to identify a manifest

      Let manifest URL be that absolute URL.

      If there is no application cache group identified by manifest URL, then create a new application cache group identified by manifest URL. Initially, it has no application caches. One will be created later in this algorithm.

      If these steps were invoked with an application cache group

      Let manifest URL be the absolute URL of the manifest used to identify the application cache group to be updated.

      If that application cache group is obsolete, then abort this instance of the application cache download process. This can happen if another instance of this algorithm found the manifest to be 404 or 410 while this algorithm was waiting in the first step above.

    2. Let cache group be the application cache group identified by manifest URL.

    3. If these steps were invoked with a master resource, then add the resource, along with the resource's Document, to cache group's list of pending master entries.

    4. If these steps were invoked with a cache host, and the status of cache group is checking or downloading, then queue a post-load task to fire a simple event named checking that is cancelable at the ApplicationCache singleton of that cache host. The default action of this event must be, if the user agent shows caching progress, the display of some sort of user interface indicating to the user that the user agent is checking to see if it can download the application.

    5. If these steps were invoked with a cache host, and the status of cache group is downloading, then also queue a post-load task to fire a simple event named downloading that is cancelable at the ApplicationCache singleton of that cache host. The default action of this event must be, if the user agent shows caching progress, the display of some sort of user interface indicating to the user the application is being downloaded.

    6. If the status of the cache group is either checking or downloading, then abort this instance of the application cache download process, as an update is already in progress.

    7. Set the status of cache group to checking.

    8. For each cache host associated with an application cache in cache group, queue a post-load task to fire a simple event that is cancelable named checking at the ApplicationCache singleton of the cache host. The default action of these events must be, if the user agent shows caching progress, the display of some sort of user interface indicating to the user that the user agent is checking for the availability of updates.

    The remainder of the steps run in parallel.

    If cache group already has an application cache in it, then this is an upgrade attempt. Otherwise, this is a cache attempt.

  3. If this is a cache attempt, then this algorithm was invoked with a cache host; queue a post-load task to fire a simple event named checking that is cancelable at the ApplicationCache singleton of that cache host. The default action of this event must be, if the user agent shows caching progress, the display of some sort of user interface indicating to the user that the user agent is checking for the availability of updates.

  4. Fetching the manifest: Fetch the resource from manifest URL with the blocking flag set, and let manifest be that resource. HTTP caching semantics should be honored for this request. The override referrer source must be null.

    Parse manifest according to the rules for parsing manifests, obtaining a list of explicit entries, fallback entries and the fallback namespaces that map to them, entries for the online whitelist, and values for the online whitelist wildcard flag and the cache mode flag.

    The MIME type of the resource is ignored — it is assumed to be text/cache-manifest. In the future, if new manifest formats are supported, the different types will probably be distinguished on the basis of the file signatures (for the current format, that is the "CACHE MANIFEST" string at the top of the file).

  5. If fetching the manifest fails due to a 404 or 410 response or equivalent, then run these substeps:

    1. Mark cache group as obsolete. This cache group no longer exists for any purpose other than the processing of Document objects already associated with an application cache in the cache group.

    2. Let task list be an empty list of tasks.

    3. For each cache host associated with an application cache in cache group, create a task to fire a simple event named obsolete that is cancelable at the ApplicationCache singleton of the cache host, and append it to task list. The default action of these events must be, if the user agent shows caching progress, the display of some sort of user interface indicating to the user that the application is no longer available for offline use.

    4. For each entry in cache group's list of pending master entries, create a task to fire a simple event that is cancelable named error (not obsolete!) at the ApplicationCache singleton of the Document for this entry, if there still is one, and append it to task list. The default action of this event must be, if the user agent shows caching progress, the display of some sort of user interface indicating to the user that the user agent failed to save the application for offline use.

    5. If cache group has an application cache whose completeness flag is incomplete, then discard that application cache.

    6. If appropriate, remove any user interface indicating that an update for this cache is in progress.

    7. Let the status of cache group be idle.

    8. For each task in task list, queue that task as a post-load task.

    9. Abort the application cache download process.

  6. Otherwise, if fetching the manifest fails in some other way (e.g. the server returns another 4xx or 5xx response or equivalent, or there is a DNS error, or the connection times out, or the user cancels the download, or the parser for manifests fails when checking the magic signature), or if the server returned a redirect, then run the cache failure steps. [[!HTTP]]

  7. If this is an upgrade attempt and the newly downloaded manifest is byte-for-byte identical to the manifest found in the newest application cache in cache group, or the server reported it as "304 Not Modified" or equivalent, then run these substeps:

    1. Let cache be the newest application cache in cache group.

    2. Let task list be an empty list of tasks.

    3. For each entry in cache group's list of pending master entries, wait for the resource for this entry to have either completely downloaded or failed.

      If the download failed (e.g. the server returns a 4xx or 5xx response or equivalent, or there is a DNS error, the connection times out, or the user cancels the download), or if the resource is labeled with the "no-store" cache directive, then create a task to fire a simple event that is cancelable named error at the ApplicationCache singleton of the Document for this entry, if there still is one, and append it to task list. The default action of this event must be, if the user agent shows caching progress, the display of some sort of user interface indicating to the user that the user agent failed to save the application for offline use.

      Otherwise, associate the Document for this entry with cache; store the resource for this entry in cache, if it isn't already there, and categorise its entry as a master entry. If applying the URL parser algorithm to the resource's URL results in a parsed URL that has a non-null fragment component, the URL used for the entry in cache must instead be the absolute URL obtained from applying the URL serialiser algorithm to the parsed URL with the exclude fragment flag set (application caches never include fragment identifiers).

    4. For each cache host associated with an application cache in cache group, create a task to fire a simple event that is cancelable named noupdate at the ApplicationCache singleton of the cache host, and append it to task list. The default action of these events must be, if the user agent shows caching progress, the display of some sort of user interface indicating to the user that the application is up to date.

    5. Empty cache group's list of pending master entries.

    6. If appropriate, remove any user interface indicating that an update for this cache is in progress.

    7. Let the status of cache group be idle.

    8. For each task in task list, queue that task as a post-load task.

    9. Abort the application cache download process.

  8. Let new cache be a newly created application cache in cache group. Set its completeness flag to incomplete.

  9. For each entry in cache group's list of pending master entries, associate the Document for this entry with new cache.

  10. Set the status of cache group to downloading.

  11. For each cache host associated with an application cache in cache group, queue a post-load task to fire a simple event that is cancelable named downloading at the ApplicationCache singleton of the cache host. The default action of these events must be, if the user agent shows caching progress, the display of some sort of user interface indicating to the user that a new version is being downloaded.

  12. Let file list be an empty list of URLs with flags.

  13. Add all the URLs in the list of explicit entries obtained by parsing manifest to file list, each flagged with "explicit entry".

  14. Add all the URLs in the list of fallback entries obtained by parsing manifest to file list, each flagged with "fallback entry".

  15. If this is an upgrade attempt, then add all the URLs of master entries in the newest application cache in cache group whose completeness flag is complete to file list, each flagged with "master entry".

  16. If any URL is in file list more than once, then merge the entries into one entry for that URL, that entry having all the flags that the original entries had.

  17. For each URL in file list, run the following steps. These steps may be run in parallel for two or more of the URLs at a time. If, while running these steps, the ApplicationCache object's abort() method sends a signal to this instance of the application cache download process algorithm, then run the cache failure steps instead.

    1. If the resource URL being processed was flagged as neither an "explicit entry" nor or a "fallback entry", then the user agent may skip this URL.

      This is intended to allow user agents to expire resources not listed in the manifest from the cache. Generally, implementors are urged to use an approach that expires lesser-used resources first.

    2. For each cache host associated with an application cache in cache group, queue a progress post-load task to fire a trusted event with the name progress, which does not bubble, which is cancelable, and which uses the ProgressEvent interface, at the ApplicationCache singleton of the cache host. The lengthComputable attribute must be set to true, the total attribute must be set to the number of files in file list, and the loaded attribute must be set to the number of files in file list that have been either downloaded or skipped so far. The default action of these events must be, if the user agent shows caching progress, the display of some sort of user interface indicating to the user that a file is being downloaded in preparation for updating the application. [[!XHR]]

    3. Fetch the resource, from the origin of the URL manifest URL, with the blocking flag set and the manual redirect flag set. If this is an upgrade attempt, then use the newest application cache in cache group as an HTTP cache, and honor HTTP caching semantics (such as expiration, ETags, and so forth) with respect to that cache. User agents may also have other caches in place that are also honored. The override referrer source must be null.

      If the resource in question is already being downloaded for other reasons then the existing download process can sometimes be used for the purposes of this step, as defined by the fetching algorithm.

      An example of a resource that might already be being downloaded is a large image on a Web page that is being seen for the first time. The image would get downloaded to satisfy the img element on the page, as well as being listed in the cache manifest. According to the rules for fetching that image only need be downloaded once, and it can be used both for the cache and for the rendered Web page.

    4. If the previous step fails (e.g. the server returns a 4xx or 5xx response or equivalent, or there is a DNS error, or the connection times out, or the user cancels the download), or if the server returned a redirect, or if the resource is labeled with the "no-store" cache directive, then run the first appropriate step from the following list: [[!HTTP]]

      If the URL being processed was flagged as an "explicit entry" or a "fallback entry"

      If these steps are being run in parallel for any other URLs in file list, then abort these steps for those other URLs. Run the cache failure steps.

      Redirects are fatal because they are either indicative of a network problem (e.g. a captive portal); or would allow resources to be added to the cache under URLs that differ from any URL that the networking model will allow access to, leaving orphan entries; or would allow resources to be stored under URLs different than their true URLs. All of these situations are bad.

      If the error was a 404 or 410 HTTP response or equivalent
      If the resource was labeled with the "no-store" cache directive

      Skip this resource. It is dropped from the cache.

      Otherwise

      Copy the resource and its metadata from the newest application cache in cache group whose completeness flag is complete, and act as if that was the fetched resource, ignoring the resource obtained from the network.

      User agents may warn the user of these errors as an aid to development.

      These rules make errors for resources listed in the manifest fatal, while making it possible for other resources to be removed from caches when they are removed from the server, without errors, and making non-manifest resources survive server-side errors.

      Except for the "no-store" directive, HTTP caching rules that would cause a file to be expired or otherwise not cached are ignored for the purposes of the application cache download process.

    5. Otherwise, the fetching succeeded. Store the resource in the new cache.

      If the user agent is not able to store the resource (e.g. because of quota restrictions), the user agent may prompt the user or try to resolve the problem in some other manner (e.g. automatically pruning content in other caches). If the problem cannot be resolved, the user agent must run the cache failure steps.

    6. If the URL being processed was flagged as an "explicit entry" in file list, then categorise the entry as an explicit entry.

    7. If the URL being processed was flagged as a "fallback entry" in file list, then categorise the entry as a fallback entry.

    8. If the URL being processed was flagged as an "master entry" in file list, then categorise the entry as a master entry.

    9. As an optimization, if the resource is an HTML or XML file whose root element is an html element with a manifest attribute whose value doesn't match the manifest URL of the application cache being processed, then the user agent should mark the entry as being foreign.

  18. For each cache host associated with an application cache in cache group, queue a progress post-load task to fire a trusted event with the name progress, which does not bubble, which is cancelable, and which uses the ProgressEvent interface, at the ApplicationCache singleton of the cache host. The lengthComputable attribute must be set to true, the total and the loaded attributes must be set to the number of files in file list. The default action of these events must be, if the user agent shows caching progress, the display of some sort of user interface indicating to the user that all the files have been downloaded. [[!XHR]]

  19. Store the list of fallback namespaces, and the URLs of the fallback entries that they map to, in new cache.

  20. Store the URLs that form the new online whitelist in new cache.

  21. Store the value of the new online whitelist wildcard flag in new cache.

  22. Store the value of the new cache mode flag in new cache.

  23. For each entry in cache group's list of pending master entries, wait for the resource for this entry to have either completely downloaded or failed.

    If the download failed (e.g. the server returns a 4xx or 5xx response or equivalent, or there is a DNS error, the connection times out, or the user cancels the download), or if the resource is labeled with the "no-store" cache directive, then run these substeps:

    1. Unassociate the Document for this entry from new cache.

    2. Queue a post-load task to fire a simple event that is cancelable named error at the ApplicationCache singleton of the Document for this entry, if there still is one. The default action of this event must be, if the user agent shows caching progress, the display of some sort of user interface indicating to the user that the user agent failed to save the application for offline use.

    3. If this is a cache attempt and this entry is the last entry in cache group's list of pending master entries, then run these further substeps:

      1. Discard cache group and its only application cache, new cache.

      2. If appropriate, remove any user interface indicating that an update for this cache is in progress.

      3. Abort the application cache download process.

    4. Otherwise, remove this entry from cache group's list of pending master entries.

    Otherwise, store the resource for this entry in new cache, if it isn't already there, and categorise its entry as a master entry.

  24. Fetch the resource from manifest URL again, with the blocking flag set, and let second manifest be that resource. HTTP caching semantics should again be honored for this request. The override referrer source must be null.

    Since caching can be honored, authors are encouraged to avoid setting the cache headers on the manifest in such a way that the user agent would simply not contact the network for this second request; otherwise, the user agent would not notice if the cache had changed during the cache update process.

  25. If the previous step failed for any reason, or if the fetching attempt involved a redirect, or if second manifest and manifest are not byte-for-byte identical, then schedule a rerun of the entire algorithm with the same parameters after a short delay, and run the cache failure steps.

  26. Otherwise, store manifest in new cache, if it's not there already, and categorise its entry as the manifest.

  27. Set the completeness flag of new cache to complete.

  28. Let task list be an empty list of tasks.

  29. If this is a cache attempt, then for each cache host associated with an application cache in cache group, create a task to fire a simple event that is cancelable named cached at the ApplicationCache singleton of the cache host, and append it to task list. The default action of these events must be, if the user agent shows caching progress, the display of some sort of user interface indicating to the user that the application has been cached and that they can now use it offline.

    Otherwise, it is an upgrade attempt. For each cache host associated with an application cache in cache group, create a task to fire a simple event that is cancelable named updateready at the ApplicationCache singleton of the cache host, and append it to task list. The default action of these events must be, if the user agent shows caching progress, the display of some sort of user interface indicating to the user that a new version is available and that they can activate it by reloading the page.

  30. If appropriate, remove any user interface indicating that an update for this cache is in progress.

  31. Set the update status of cache group to idle.

  32. For each task in task list, queue that task as a post-load task.

The cache failure steps are as follows:

  1. Let task list be an empty list of tasks.

  2. For each entry in cache group's list of pending master entries, run the following further substeps. These steps may be run in parallel for two or more entries at a time.

    1. Wait for the resource for this entry to have either completely downloaded or failed.

    2. Unassociate the Document for this entry from its application cache, if it has one.

    3. Create a task to fire a simple event that is cancelable named error at the ApplicationCache singleton of the Document for this entry, if there still is one, and append it to task list. The default action of these events must be, if the user agent shows caching progress, the display of some sort of user interface indicating to the user that the user agent failed to save the application for offline use.

  3. For each cache host still associated with an application cache in cache group, create a task to fire a simple event that is cancelable named error at the ApplicationCache singleton of the cache host, and append it to task list. The default action of these events must be, if the user agent shows caching progress, the display of some sort of user interface indicating to the user that the user agent failed to save the application for offline use.

  4. Empty cache group's list of pending master entries.

  5. If cache group has an application cache whose completeness flag is incomplete, then discard that application cache.

  6. If appropriate, remove any user interface indicating that an update for this cache is in progress.

  7. Let the status of cache group be idle.

  8. If this was a cache attempt, discard cache group altogether.

  9. For each task in task list, queue that task as a post-load task.

  10. Abort the application cache download process.

Attempts to fetch resources as part of the application cache download process may be done with cache-defeating semantics, to avoid problems with stale or inconsistent intermediary caches.

User agents may invoke the application cache download process, in the background, for any application cache group, at any time (with no cache host). This allows user agents to keep caches primed and to update caches even before the user visits a site.

Each Document has a list of pending application cache download process tasks that is used to delay events fired by the algorithm above until the document's load event has fired. When the Document is created, the list must be empty.

When the steps above say to queue a post-load task task, where task is a task that dispatches an event on a target ApplicationCache object target, the user agent must run the appropriate steps from the following list:

If target's node document is ready for post-load tasks

Queue the task task.

Otherwise

Add task to target's node document's list of pending application cache download process tasks.

When the steps above say to queue a progress post-load task task, where task is a task that dispatches an event on a target ApplicationCache object target, the user agent must run the following steps:

  1. If there is a task in target's node document's list of pending application cache download process tasks that is labeled as a progress task, then remove that task from the list.

  2. Label task as a progress task.

  3. Queue a post-load task task.

The task source for these tasks is the networking task source.

The application cache selection algorithm

When the application cache selection algorithm algorithm is invoked with a Document document and optionally a manifest URL manifest URL, the user agent must run the first applicable set of steps from the following list:

If there is a manifest URL, and document was loaded from an application cache, and the URL of the manifest of that cache's application cache group is not the same as manifest URL

Mark the entry for the resource from which document was taken in the application cache from which it was loaded as foreign.

Restart the current navigation from the top of the navigation algorithm, undoing any changes that were made as part of the initial load (changes can be avoided by ensuring that the step to update the session history with the new page is only ever completed after this application cache selection algorithm is run, though this is not required).

The navigation will not result in the same resource being loaded, because "foreign" entries are never picked during navigation.

User agents may notify the user of the inconsistency between the cache manifest and the document's own metadata, to aid in application development.

If document was loaded from an application cache, and that application cache still exists (it is not now obsolete)

Associate document with the application cache from which it was loaded. Invoke, in the background, the application cache download process for that application cache's application cache group, with document as the cache host.

If document was loaded using HTTP GET or equivalent, and, there is a manifest URL, and manifest URL has the same origin as document

Invoke, in the background, the application cache download process for manifest URL, with document as the cache host and with the resource from which document was parsed as the master resource.

If there are relevant application caches that are identified by a URL with the same origin as the URL of document, and that have this URL as one of their entries, excluding entries marked as foreign, then the user agent should use the most appropriate application cache of those that match as an HTTP cache for any subresource loads. User agents may also have other caches in place that are also honored.

Otherwise

The Document is not associated with any application cache.

If there was a manifest URL, the user agent may report to the user that it was ignored, to aid in application development.

Changes to the networking model

When a cache host is associated with an application cache whose completeness flag is complete, any and all loads for resources related to that cache host other than those for child browsing contexts must go through the following steps instead of immediately invoking the mechanisms appropriate to that resource's scheme:

  1. If the resource is not to be fetched using the HTTP GET mechanism or equivalent, or if applying the URL parser algorithm to both its URL and the application cache's manifest's URL results in two parsed URLs with different scheme components, then fetch the resource normally and abort these steps.

  2. If the resource's URL is a master entry, the manifest, an explicit entry, or a fallback entry in the application cache, then get the resource from the cache (instead of fetching it), and abort these steps.

  3. If there is an entry in the application cache's online whitelist that has the same origin as the resource's URL and that is a prefix match for the resource's URL, then fetch the resource normally and abort these steps.

  4. If the resource's URL has the same origin as the manifest's URL, and there is a fallback namespace f in the application cache that is a prefix match for the resource's URL, then:

    Fetch the resource normally. If this results in a redirect to a resource with another origin (indicative of a captive portal), or a 4xx or 5xx status code or equivalent, or if there were network errors (but not if the user canceled the download), then instead get, from the cache, the resource of the fallback entry corresponding to the fallback namespace f. Abort these steps.

  5. If the application cache's online whitelist wildcard flag is open, then fetch the resource normally and abort these steps.

  6. Fail the resource load as if there had been a generic network error.

The above algorithm ensures that so long as the online whitelist wildcard flag is blocking, resources that are not present in the manifest will always fail to load (at least, after the application cache has been primed the first time), making the testing of offline applications simpler.

Expiring application caches

As a general rule, user agents should not expire application caches, except on request from the user, or after having been left unused for an extended period of time.

Application caches and cookies have similar implications with respect to privacy (e.g. if the site can identify the user when providing the cache, it can store data in the cache that can be used for cookie resurrection). Implementors are therefore encouraged to expose application caches in a manner related to HTTP cookies, allowing caches to be expunged together with cookies and other origin-specific data.

For example, a user agent could have a "delete site-specific data" feature that clears all cookies, application caches, local storage, databases, etc, from an origin all at once.

Disk space

User agents should consider applying constraints on disk usage of application caches, and care should be taken to ensure that the restrictions cannot be easily worked around using subdomains.

User agents should allow users to see how much space each domain is using, and may offer the user the ability to delete specific application caches.

For predictability, quotas should be based on the uncompressed size of data stored.

How quotas are presented to the user is not defined by this specification. User agents are encouraged to provide features such as allowing a user to indicate that certain sites are trusted to use more than the default quota, e.g. by presenting a non-modal user interface while a cache is being updated, or by having an explicit whitelist in the user agent's configuration interface.

Security concerns with offline applications caches

This section is non-normative.

The main risk introduced by offline application caches is that an injection attack can be elevated into persistent site-wide page replacement. This attack involves using an injection vulnerability to upload two files to the victim site. The first file is an application cache manifest consisting of just a fallback entry pointing to the second file, which is an HTML page whose manifest is declared as that first file. Once the user has been directed to that second file, all subsequent accesses to any file covered by the given fallback namespace while either the user or the site is offline will instead show that second file. Targetted denial-of-service attacks or cookie bombing attacks (where the client is made to send so many cookies that the server refuses to process the request) can be used to ensure that the site appears offline.

To mitigate this, manifests can only specify fallbacks that are in the same path as the manifest itself. This means that a content injection upload vulnerability in a particular directory on a server can only be escalated to a take-over of that directory and its subdirectories. If there is no way to inject a file into the root directory, the entire site cannot be taken over.

If a site has been attacked in this way, simply removing the offending manifest might eventually clear the problem, since the next time the manifest is updated, a 404 error will be seen, and the user agent will clear the cache. "Eventually" is the key word here, however; while the attack on the user or server is ongoing, such that connections from an affected user to the affected site are blocked, the user agent will simply assume that the user is offline and will continue to use the hostile manifest. Unfortunately, if a cookie bombing attack has also been used, merely removing the manifest is insufficient; in addition, the server has to be configured to return a 404 or 410 response instead of the 413 "Request Entity Too Large" response.

TLS does not inherently protect a site from this attack, since the attack relies on content being served from the server itself. Not using application caches also does not prevent this attack, since the attack relies on an attacker-provided manifest.

Application cache API

[Exposed=(Window,SharedWorker)]
interface ApplicationCache : EventTarget {

  // update status
  const unsigned short UNCACHED = 0;
  const unsigned short IDLE = 1;
  const unsigned short CHECKING = 2;
  const unsigned short DOWNLOADING = 3;
  const unsigned short UPDATEREADY = 4;
  const unsigned short OBSOLETE = 5;
  readonly attribute unsigned short status;

  // updates
  void update();
  void abort();
  void swapCache();

  // events
  attribute EventHandler onchecking;
  attribute EventHandler onerror;
  attribute EventHandler onnoupdate;
  attribute EventHandler ondownloading;
  attribute EventHandler onprogress;
  attribute EventHandler onupdateready;
  attribute EventHandler oncached;
  attribute EventHandler onobsolete;
};
cache = window . applicationCache

(In a window.) Returns the ApplicationCache object that applies to the active document of that Window.

cache = self . applicationCache

(In a shared worker.) Returns the ApplicationCache object that applies to the current shared worker.

cache . status

Returns the current status of the application cache, as given by the constants defined below.

cache . update()

Invokes the application cache download process.

Throws an InvalidStateError exception if there is no application cache to update.

Calling this method is not usually necessary, as user agents will generally take care of updating application caches automatically.

The method can be useful in situations such as long-lived applications. For example, a Web mail application might stay open in a browser tab for weeks at a time. Such an application could want to test for updates each day.

cache . abort()

Cancels the application cache download process.

This method is intended to be used by Web application showing their own caching progress UI, in case the user wants to stop the update (e.g. because bandwidth is limited).

cache . swapCache()

Switches to the most recent application cache, if there is a newer one. If there isn't, throws an InvalidStateError exception.

This does not cause previously-loaded resources to be reloaded; for example, images do not suddenly get reloaded and style sheets and scripts do not get reparsed or reevaluated. The only change is that subsequent requests for cached resources will obtain the newer copies.

The updateready event will fire before this method can be called. Once it fires, the Web application can, at its leisure, call this method to switch the underlying cache to the one with the more recent updates. To make proper use of this, applications have to be able to bring the new features into play; for example, reloading scripts to enable new features.

An easier alternative to swapCache() is just to reload the entire page at a time suitable for the user, using location.reload().

There is a one-to-one mapping from cache hosts to ApplicationCache objects. The applicationCache attribute on Window objects must return the ApplicationCache object associated with the Window object's active document. The applicationCache attribute on SharedWorkerGlobalScope objects must return the ApplicationCache object associated with the worker.

A Window or SharedWorkerGlobalScope object has an associated ApplicationCache object even if that cache host has no actual application cache.

The status attribute, on getting, must return the current state of the application cache that the ApplicationCache object's cache host is associated with, if any. This must be the appropriate value from the following list:

UNCACHED (numeric value 0)

The ApplicationCache object's cache host is not associated with an application cache at this time.

IDLE (numeric value 1)

The ApplicationCache object's cache host is associated with an application cache whose application cache group's update status is idle, and that application cache is the newest cache in its application cache group, and the application cache group is not marked as obsolete.

CHECKING (numeric value 2)

The ApplicationCache object's cache host is associated with an application cache whose application cache group's update status is checking.

DOWNLOADING (numeric value 3)

The ApplicationCache object's cache host is associated with an application cache whose application cache group's update status is downloading.

UPDATEREADY (numeric value 4)

The ApplicationCache object's cache host is associated with an application cache whose application cache group's update status is idle, and whose application cache group is not marked as obsolete, but that application cache is not the newest cache in its group.

OBSOLETE (numeric value 5)

The ApplicationCache object's cache host is associated with an application cache whose application cache group is marked as obsolete.

If the update() method is invoked, the user agent must invoke the application cache download process, in the background, for the application cache group of the application cache with which the ApplicationCache object's cache host is associated, but without giving that cache host to the algorithm. If there is no such application cache, or if its application cache group is marked as obsolete, then the method must throw an InvalidStateError exception instead.

If the abort() method is invoked, the user agent must send a signal to the current application cache download process for the application cache group of the application cache with which the ApplicationCache object's cache host is associated, if any. If there is no such application cache, or it does not have a current application cache download process, then do nothing.

If the swapCache() method is invoked, the user agent must run the following steps:

  1. Check that ApplicationCache object's cache host is associated with an application cache. If it is not, then throw an InvalidStateError exception and abort these steps.

  2. Let cache be the application cache with which the ApplicationCache object's cache host is associated. (By definition, this is the same as the one that was found in the previous step.)

  3. If cache's application cache group is marked as obsolete, then unassociate the ApplicationCache object's cache host from cache and abort these steps. (Resources will now load from the network instead of the cache.)

  4. Check that there is an application cache in the same application cache group as cache whose completeness flag is complete and that is newer than cache. If there is not, then throw an InvalidStateError exception and abort these steps.

  5. Let new cache be the newest application cache in the same application cache group as cache whose completeness flag is complete.

  6. Unassociate the ApplicationCache object's cache host from cache and instead associate it with new cache.

The following are the event handlers (and their corresponding event handler event types) that must be supported, as event handler IDL attributes, by all objects implementing the ApplicationCache interface:

Event handler Event handler event type
onchecking checking
onerror error
onnoupdate noupdate
ondownloading downloading
onprogress progress
onupdateready updateready
oncached cached
onobsolete obsolete

Browser state

[NoInterfaceObject, Exposed=(Window,Worker)]
interface NavigatorOnLine {
  readonly attribute boolean onLine;
};
window . navigator . onLine

Returns false if the user agent is definitely offline (disconnected from the network). Returns true if the user agent might be online.

The events online and offline are fired when the value of this attribute changes.

The navigator.onLine attribute must return false if the user agent will not contact the network when the user follows links or when a script requests a remote page (or knows that such an attempt would fail), and must return true otherwise.

When the value that would be returned by the navigator.onLine attribute of a Window or WorkerGlobalScope changes from true to false, the user agent must queue a task to fire a simple event named offline at the Window or WorkerGlobalScope object.

On the other hand, when the value that would be returned by the navigator.onLine attribute of a Window or WorkerGlobalScope changes from false to true, the user agent must queue a task to fire a simple event named online at the Window or WorkerGlobalScope object.

The task source for these tasks is the networking task source.

This attribute is inherently unreliable. A computer can be connected to a network without having Internet access.

In this example, an indicator is updated as the browser goes online and offline.

<!DOCTYPE HTML>
<html>
 <head>
  <title>Online status</title>
  <script>
   function updateIndicator() {
     document.getElementById('indicator').textContent = navigator.onLine ? 'online' : 'offline';
   }
  </script>
 </head>
 <body onload="updateIndicator()" ononline="updateIndicator()" onoffline="updateIndicator()">
  <p>The network is: <span id="indicator">(state unknown)</span>
 </body>
</html>