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algorithm -- LEFT-TO-RIGHT OVERRIDE (202D) or RIGHT-TO-LEFT OVERRIDE
(hexadecimal 202E). The POP DIRECTIONAL FORMATTING (hexadecimal 202C)
character ends either bidirectional override.
Note. Recall that conflicts can arise if the dir attribute is used on inline elements
(including BDO) concurrently with the corresponding [UNICODE] [p.355] formatting
characters.
Bidirectionality and character encoding According to [RFC1555] [p.354] and
[RFC1556] [p.354] , there are special conventions for the use of "charset" parameter
values to indicate bidirectional treatment in MIME mail, in particular to distinguish
between visual, implicit, and explicit directionality. The parameter value
"ISO-8859-8" (for Hebrew) denotes visual encoding, "ISO-8859-8-i" denotes implicit
bidirectionality, and "ISO-8859-8-e" denotes explicit directionality.
Because HTML uses the Unicode bidirectionality algorithm, conforming
documents encoded using ISO 8859-8 must be labeled as "ISO-8859-8-i". Explicit
directional control is also possible with HTML, but cannot be expressed with ISO
8859-8, so "ISO-8859-8-e" should not be used.
The value "ISO-8859-8" implies that the document is formatted visually, misusing
some markup (such as TABLE with right alignment and no line wrapping) to ensure
reasonable display on older user agents that do not handle bidirectionality. Such
documents do not conform to the present specification. If necessary, they can be
made to conform to the current specification (and at the same time will be displayed
correctly on older user agents) by adding BDO markup where necessary. Contrary to
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what is said in [RFC1555] [p.354] and [RFC1556] [p.354] , ISO-8859-6 (Arabic) is
not visual ordering.
8.2.5 Character references for directionality and joining
control
Since ambiguities sometimes arise as to the directionality of certain characters (e.g.,
punctuation), the [UNICODE] [p.355] specification includes characters to enable their
proper resolution. Also, Unicode includes some characters to control joining behavior
where this is necessary (e.g., some situations with Arabic letters). HTML 4 includes
character references [p.299] for these characters.
The following DTD excerpt presents some of the directional entities:
The zwnj entity is used to block joining behavior in contexts where joining will
occur but shouldn t. The zwj entity does the opposite; it forces joining when it
wouldn t occur but should. For example, the Arabic letter "HEH" is used to
abbreviate "Hijri", the name of the Islamic calendar system. Since the isolated form
of "HEH" looks like the digit five as employed in Arabic script (based on Indic digits),
in order to prevent confusing "HEH" as a final digit five in a year, the initial form of
"HEH" is used. However, there is no following context (i.e., a joining letter) to which
the "HEH" can join. The zwj character provides that context.
Similarly, in Persian texts, there are cases where a letter that normally would join a
subsequent letter in a cursive connection should not. The character zwnj is used to
block joining in such cases.
The other characters, lrm and rlm, are used to force directionality of directionally
neutral characters. For example, if a double quotation mark comes between an
Arabic (right-to-left) and a Latin (left-to-right) letter, the direction of the quotation
mark is not clear (is it quoting the Arabic text or the Latin text?). The lrm and rlm
characters have a directional property but no width and no word/line break property.
Please consult [UNICODE] [p.355] for more details.
Mirrored character glyphs. In general, the bidirectional algorithm does not mirror
character glyphs but leaves them unaffected. An exception are characters such as
parentheses (see [UNICODE] [p.355] , table 4-7). In cases where mirroring is
desired, for example for Egyptian Hieroglyphs, Greek Bustrophedon, or special
design effects, this should be controlled with styles.
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8.2.6 The effect of style sheets on bidirectionality
In general, using style sheets to change an element s visual rendering from
block-level to inline or vice-versa is straightforward. However, because the
bidirectional algorithm relies on the inline/block-level distinction [p.83] , special care
must be taken during the transformation.
When an inline element that does not have a dir attribute is transformed to the [ Pobierz całość w formacie PDF ]