How to handle bidirectional content in HTML.

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.

The bdi element

Categories:
Flow content.
Phrasing content.
Palpable content.
Contexts in which this element can be used:
Where phrasing content is expected.
Content model:
Phrasing content.
Tag omission in text/html:
Neither tag is omissible.
Content attributes:
Global attributes
Also, the dir global attribute has special semantics on this element.
Allowed ARIA role attribute values:
Any role value.
Allowed ARIA State and Property Attributes:
Global aria-* attributes
Any aria-* attributes applicable to the allowed roles.
DOM interface:
Uses HTMLElement.

The bdi element represents a span of text that is to be isolated from its surroundings for the purposes of bidirectional text formatting. [[!BIDI]]

The dir global attribute defaults to auto on this element (it never inherits from the parent element like with other elements).

This element has rendering requirements involving the bidirectional algorithm.

This element is especially useful when embedding user-generated content with an unknown directionality.

In this example, usernames are shown along with the number of posts that the user has submitted. If the bdi element were not used, the username of the Arabic user would end up confusing the text (the bidirectional algorithm would put the colon and the number "3" next to the word "User" rather than next to the word "posts").

<ul>
 <li>User <bdi>jcranmer</bdi>: 12 posts.
 <li>User <bdi>hober</bdi>: 5 posts.
 <li>User <bdi>إيان</bdi>: 3 posts.
</ul>
When using the bdi element, the username acts as expected.
If the bdi element were to be replaced by a b element, the username would confuse the bidirectional algorithm and the third bullet would end up saying "User 3 :", followed by the Arabic name (right-to-left), followed by "posts" and a period.

The bdo element

Categories:
Flow content.
Phrasing content.
Palpable content.
Contexts in which this element can be used:
Where phrasing content is expected.
Content model:
Phrasing content.
Tag omission in text/html:
Neither tag is omissible.
Content attributes:
Global attributes
Also, the dir global attribute has special semantics on this element.
Allowed ARIA role attribute values:
Any role value.
Allowed ARIA State and Property Attributes:
Global aria-* attributes
Any aria-* attributes applicable to the allowed roles.
DOM interface:
Uses HTMLElement.

The bdo element represents explicit text directionality formatting control for its children. It allows authors to override the Unicode bidirectional algorithm by explicitly specifying a direction override. [[!BIDI]]

Authors must specify the dir attribute on this element, with the value ltr to specify a left-to-right override and with the value rtl to specify a right-to-left override. The auto value must not be specified.

This element has rendering requirements involving the bidirectional algorithm.

The dir attribute

The dir attribute specifies the element's text directionality. The attribute is an enumerated attribute with the following keywords and states:

The ltr keyword, which maps to the ltr state

Indicates that the contents of the element are explicitly directionally isolated left-to-right text.

The rtl keyword, which maps to the rtl state

Indicates that the contents of the element are explicitly directionally isolated right-to-left text.

The auto keyword, which maps to the auto state

Indicates that the contents of the element are explicitly directionally isolated text, but that the direction is to be determined programmatically using the contents of the element (as described below).

The heuristic used by this state is very crude (it just looks at the first character with a strong directionality, in a manner analogous to the Paragraph Level determination in the bidirectional algorithm). Authors are urged to only use this value as a last resort when the direction of the text is truly unknown and no better server-side heuristic can be applied. [[!BIDI]]

For textarea and pre elements, the heuristic is applied on a per-paragraph level.

The attribute has no invalid value default and no missing value default.

The directionality of an element (any element, not just an HTML element) is either 'ltr' or 'rtl', and is determined as per the first appropriate set of steps from the following list:

If the element's dir attribute is in the ltr state
If the element is a root element and the dir attribute is not in a defined state (i.e. it is not present or has an invalid value)
If the element is an input element whose type attribute is in the Telephone state, and the dir attribute is not in a defined state (i.e. it is not present or has an invalid value)

The directionality of the element is 'ltr'.

If the element's dir attribute is in the rtl state

The directionality of the element is 'rtl'.

If the element is an input element whose type attribute is in the Text, Search, Telephone, URL, or E-mail state, and the dir attribute is in the auto state
If the element is a textarea element and the dir attribute is in the auto state

If the element's value contains a character of bidirectional character type AL or R, and there is no character of bidirectional character type L anywhere before it in the element's value, then the directionality of the element is 'rtl'. [[!BIDI]]

Otherwise, if the element's value is not the empty string, or if the element is a root element, the directionality of the element is 'ltr'.

Otherwise, the directionality of the element is the same as the element's parent element's directionality.

If the element's dir attribute is in the auto state
If the element is a bdi element and the dir attribute is not in a defined state (i.e. it is not present or has an invalid value)

Find the first character in tree order that matches the following criteria:

  • The character is from a Text node that is a descendant of the element whose directionality is being determined.

  • The character is of bidirectional character type L, AL, or R. [[!BIDI]]

  • The character is not in a Text node that has an ancestor element that is a descendant of the element whose directionality is being determined and that is either:

If such a character is found and it is of bidirectional character type AL or R, the directionality of the element is 'rtl'.

If such a character is found and it is of bidirectional character type L, the directionality of the element is 'ltr'.

Otherwise, if the element is a root element, the directionality of the element is 'ltr'.

Otherwise, the directionality of the element the same as the element's parent element's directionality.

If the element has a parent element and the dir attribute is not in a defined state (i.e. it is not present or has an invalid value)

The directionality of the element is the same as the element's parent element's directionality.

Since the dir attribute is only defined for HTML elements, it cannot be present on elements from other namespaces. Thus, elements from other namespaces always just inherit their directionality from their parent element, or, if they don't have one, default to 'ltr'.

This attribute has rendering requirements involving the bidirectional algorithm.

The directionality of an attribute of an HTML element, which is used when the text of that attribute is to be included in the rendering in some manner, is determined as per the first appropriate set of steps from the following list:

If the attribute is a directionality-capable attribute and the element's dir attribute is in the auto state

Find the first character (in logical order) of the attribute's value that is of bidirectional character type L, AL, or R. [[!BIDI]]

If such a character is found and it is of bidirectional character type AL or R, the directionality of the attribute is 'rtl'.

Otherwise, the directionality of the attribute is 'ltr'.

Otherwise
The directionality of the attribute is the same as the element's directionality.

The following attributes are directionality-capable attributes:

document . dir [ = value ]

Returns the html element's dir attribute's value, if any.

Can be set, to either "ltr", "rtl", or "auto" to replace the html element's dir attribute's value.

If there is no html element, returns the empty string and ignores new values.

The dir IDL attribute on an element must reflect the dir content attribute of that element, limited to only known values.

The dir IDL attribute on Document objects must reflect the dir content attribute of the html element, if any, limited to only known values. If there is no such element, then the attribute must return the empty string and do nothing on setting.

Authors are strongly encouraged to use the dir attribute to indicate text direction rather than using CSS, since that way their documents will continue to render correctly even in the absence of CSS (e.g. as interpreted by search engines).

This markup fragment is of an IM conversation.

<p dir=auto class="u1"><b><bdi>Student</bdi>:</b> How do you write "What's your name?" in Arabic?</p>
<p dir=auto class="u2"><b><bdi>Teacher</bdi>:</b> ما اسمك؟</p>
<p dir=auto class="u1"><b><bdi>Student</bdi>:</b> Thanks.</p>
<p dir=auto class="u2"><b><bdi>Teacher</bdi>:</b> That's written "شكرًا".</p>
<p dir=auto class="u2"><b><bdi>Teacher</bdi>:</b> Do you know how to write "Please"?</p>
<p dir=auto class="u1"><b><bdi>Student</bdi>:</b> "من فضلك", right?</p>

Given a suitable style sheet and the default alignment styles for the p element, namely to align the text to the start edge of the paragraph, the resulting rendering could be as follows:

Each paragraph rendered as a separate block, with the paragraphs left-aligned except the second paragraph and the last one, which would be right aligned, with the usernames ('Student' and 'Teacher' in this example) flush right, with a colon to their left, and the text first to the left of that.

As noted earlier, the auto value is not a panacea. The final paragraph in this example is misinterpreted as being right-to-left text, since it begins with an Arabic character, which causes the "right?" to be to the left of the Arabic text.

Authoring conformance criteria for bidirectional-algorithm formatting characters

Text content in HTML elements with Text nodes in their contents, and text in attributes of HTML elements that allow free-form text, may contain characters in the ranges U+202A to U+202E and U+2066 to U+2069 (the bidirectional-algorithm formatting characters). However, the use of these characters is restricted so that any embedding or overrides generated by these characters do not start and end with different parent elements, and so that all such embeddings and overrides are explicitly terminated by a U+202C POP DIRECTIONAL FORMATTING character. This helps reduce incidences of text being reused in a manner that has unforeseen effects on the bidirectional algorithm. [[!BIDI]]

The aforementioned restrictions are defined by specifying that certain parts of documents form bidirectional-algorithm formatting character ranges, and then imposing a requirement on such ranges.

The strings resulting from applying the following algorithm to an HTML element element are bidirectional-algorithm formatting character ranges:

  1. Let output be an empty list of strings.

  2. Let string be an empty string.

  3. Let node be the first child node of element, if any, or null otherwise.

  4. Loop: If node is null, jump to the step labeled end.

  5. Process node according to the first matching step from the following list:

    If node is a Text node

    Append the text data of node to string.

    If node is a br element
    If node is an HTML element that is flow content but that is not also phrasing content

    If string is not the empty string, push string onto output, and let string be empty string.

    Otherwise
    Do nothing.

  6. Let node be node's next sibling, if any, or null otherwise.

  7. Jump to the step labeled loop.

  8. End: If string is not the empty string, push string onto output.

  9. Return output as the bidirectional-algorithm formatting character ranges.

The value of a namespace-less attribute of an HTML element is a bidirectional-algorithm formatting character range.

Any strings that, as described above, are bidirectional-algorithm formatting character ranges must match the string production in the following ABNF, the character set for which is Unicode. [[!ABNF]]

string        = *( plaintext ( embedding / override / isolation ) ) plaintext
embedding     = ( lre / rle ) string pdf
override      = ( lro / rlo ) string pdf
isolation     = ( lri / rli / fsi ) string pdi
lre           = %x202A ; U+202A LEFT-TO-RIGHT EMBEDDING
rle           = %x202B ; U+202B RIGHT-TO-LEFT EMBEDDING
lro           = %x202D ; U+202D LEFT-TO-RIGHT OVERRIDE
rlo           = %x202E ; U+202E RIGHT-TO-LEFT OVERRIDE
pdf           = %x202C ; U+202C POP DIRECTIONAL FORMATTING
lri           = %x2066 ; U+2066 LEFT-TO-RIGHT ISOLATE
rli           = %x2067 ; U+2067 RIGHT-TO-LEFT ISOLATE
fsi           = %x2068 ; U+2068 FIRST STRONG ISOLATE
pdi           = %x2069 ; U+2069 POP DIRECTIONAL ISOLATE
plaintext     = *( %x0000-2029 / %x202F-2065 / %x206A-10FFFF )
                ; any string with no bidirectional-algorithm formatting characters

While the U+2069 POP DIRECTIONAL ISOLATE character implicitly also ends open embeddings and overrides, text that relies on this implicit scope closure is not conforming to this specification. All strings of embeddings, overrides, and isolations need to be explicitly terminated to conform to this section's requirements.

Authors are encouraged to use the dir attribute, the bdo element, and the bdi element, rather than maintaining the bidirectional-algorithm formatting characters manually. The bidirectional-algorithm formatting characters interact poorly with CSS.

User agent conformance criteria

User agents must implement the Unicode bidirectional algorithm to determine the proper ordering of characters when rendering documents and parts of documents. [[!BIDI]]

The mapping of HTML to the Unicode bidirectional algorithm must be done in one of three ways. Either the user agent must implement CSS, including in particular the CSS 'unicode-bidi', 'direction', and 'content' properties, and must have, in its user agent style sheet, the rules using those properties given in this specification's rendering section, or, alternatively, the user agent must act as if it implemented just the aforementioned properties and had a user agent style sheet that included all the aforementioned rules, but without letting style sheets specified in documents override them, or, alternatively, the user agent must implement another styling language with equivalent semantics. [[!CSSWM]] [[!CSSGC]]

The following elements and attributes have requirements defined by the rendering section that, due to the requirements in this section, are requirements on all user agents (not just those that support the suggested default rendering):