15. XHTML Bi-directional Text Attribute Module

Contents

15.1. Bi-directional Text Collection
- 15.1.1. Inheritance of text direction information
- 15.1.2. The effect of style sheets on bidirectionality

This section is normative.

The Bi-directional Text module defines the Bi-directional attribute collection.

15.1. Bi-directional Text Collection

dir = "ltr|rtl|lro|rlo"

This attribute allows the author to specify the direction of the element's text content. This direction affects the display of characters as defined in Unicode Standard Annex #9: The Bidirectional Algorithm [UAX9], and defines directional properties of text as defined by CSS2 [CSS2]. The default value of this attribute is ltr. Possible values are:

ltr: Left-to-right text. The effect of this attribute is defined by the CSS2 rule:
```
*[dir="ltr"] { unicode-bidi: embed; direction: ltr}
```
rtl: Right-to-left text. The effect of this attribute is defined by the CSS2 rule:
```
*[dir="rtl"] { unicode-bidi: embed; direction: rtl}
```
lro: Left-to-right override. The effect of this attribute is defined by the CSS2 rule:
```
*[dir="lro"] { unicode-bidi: bidi-override; direction: ltr}
```
rlo: Right-to-left override. The effect of this attribute is defined by the CSS2 rule:
```
*[dir="rlo"] { unicode-bidi: bidi-override; direction: rtl}
```

Example

<p>
The Hebrew word for "Hebrew" is 
<span xml:lang="he">&#x5e2;&#x5d1;&#x5e8;&#x5d9;&#x5ea;</span>,
but since Hebrew letters have intrinsic right-to-left directionality,
I had to type the word starting from the letter "&#x5e2;",
i.e. <span xml:lang="he" dir="lro">&#x5e2;&#x5d1;&#x5e8;&#x5d9;&#x5ea;</span>.
</p>

This might display as an image of the example

15.1.1. Inheritance of text direction information

The Unicode bidirectional algorithm requires a base text direction for text blocks. To specify the base direction of a block-level element, set the element's dir attribute. The default value of the dir attribute is ltr (left-to-right text).

When the dir attribute is set for a block-level element, it remains in effect for the duration of the element and any nested block-level elements. Setting the dir attribute on a nested element overrides the inherited value.

To set the base text direction for an entire document, set the dir attribute on the html element.

Example

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 2.0//EN"
   "http://www.w3.org/MarkUp/DTD/xhtml2.dtd">
<html xmlns="http://www.w3.org/2002/06/xhtml2/" dir="rtl">
<head>
<title><em>...a right-to-left title...</em></title>
</head>
<body>
<em>...right-to-left text...</em>
<p dir="ltr"><em>...left-to-right text...</em></p>
<p><em>...right-to-left text again...</em></p>
</body>
</html>

Inline-level elements, on the other hand, do not inherit the dir attribute. This means that an inline element without a dir attribute does not open an additional level of embedding with respect to the bidirectional algorithm.

An element is considered to be block-level if its presentation, when expressed in [CSS2], is display: block and inline-level if its presentation, when expressed in [CSS2], is display: inline.

15.1.2. The effect of style sheets on bidirectionality

In general, using style sheets (such as [CSS2] to change an element's visual rendering from the equivalent of display: block to display: inline or vice-versa is straightforward. However, because the bidirectional algorithm relies on the inline/block-level distinction, special care must be taken during the transformation.

When an inline-level element that does not have a dir attribute is transformed to a block-level element by a style sheet, it inherits the dir attribute from its closest parent block-level element to define the base direction of the block.

When a block-level element that does not have a dir attribute is transformed to an inline-level element by a style sheet, the resulting presentation should be equivalent, in terms of bidirectional formatting, to the formatting obtained by explicitly adding a dir attribute (assigned the inherited value) to the transformed element.

Implementation: RELAX NG