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RE: bidi-override scope in CSS2 and CSS 2.1

From: Richard Ishida <ishida@w3.org>
Date: Mon, 18 Sep 2006 17:11:03 +0100
To: "'Paul Nelson (ATC)'" <paulnel@winse.microsoft.com>, "'Martin Duerst'" <duerst@it.aoyama.ac.jp>, "'Bert Bos'" <bert@w3.org>, "'Steven Pemberton'" <steven.pemberton@cwi.nl>
Cc: <www-style@w3.org>, <public-i18n-core@w3.org>, "'Masayasu Ishikawa'" <mimasa@w3.org>, <public-i18n-core@w3.org>
Message-ID: <010201c6db3d$0b341b10$6501a8c0@w3cishida>

I'm unclear about the expected behaviour for bidi overrides set in CSS and
XHTML 2. And would like to get some clarifications. Some of these questions
relate more specifically to XHTML2, others to CSS.

Suppose we have the following code in and XML application such as XHTML2:

<div dir="rlo">Some inline text 
and more inline text on another line 
<p>A paragraph of text</p>
Some more <em>inline</em> text

and the following CSS style in the same document:
*[dir="lro"] { unicode-bidi: bidi-override; direction: ltr}
*[dir="rlo"] { unicode-bidi: bidi-override; direction: rtl}

[Question 1 (XHTML2)]
Do we need the CSS declarations to effect the expected presentation in

I thought we would, but the XHTML2 spec {1} says "The effect of this
attribute is defined by the CSS2 rule:", which makes me think that an
implementation must provide this behaviour whether or not the CSS rules are
explicitly stated (as is the case for HTML4.01).

[Question2 (XHTML2)]
If XHTML2 does require the presence of the CSS, there seems to be an
inconsistency in terms of the inheritance of the override behaviour.

In section 15.1.1 of XHTML2 it says "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."

On the other hand, the CSS2 specification {2} says that "If the element is
inline-level or a block-level element that contains only inline-level
elements, this creates an override."

This leads me to conclude that for the code mentioned above I'd expect to
see the following display according to the XHTML2 spec:

txet enilni emoS
enil rehtona no txet enilne erom dna
txet fo hpargarap A
txet enilni erom emoS

and the following according to the CSS2 spec:

Some inline text 
and more inline text on another line 
A paragraph of text
Some more inline text

This seems to be at odds.  Am I correct?

[Question 3 (CSS)]
Whereas the CSS2 spec {2} says "If the element is inline-level or a
block-level element that contains only inline-level elements, this creates
an override.", the CSS2.1 spec {3} says "For block-level, table-cell,
table-caption, or inline-block elements this creates an override for
inline-level descendents not within another block-level, table-cell,
table-caption, or inline-block element."

I take this to mean that with CSS2 the result for the code at the top of the
page would be:

Some inline text 
and more inline text on another line 
A paragraph of text
Some more inline text

whereas with CSS 2.1 the result would be:

txet enilni emoS
enil rehtona no txet enilne erom dna
A paragraph of text
txet enilni erom emoS

Is this correct?

[Question 4 (XHTML]
If XHTML does in fact rely on CSS declarations to cause the appropriate
display of overridden text, then how does it square the fact that the
behaviour will change, depending on whether people use CSS2 or CSS2.1
enabled implementations?

[Question 5 (CSS)]
Both CSS2 and CSS2.1 say "The final order of characters in each block-level
element is the same as if the bidi control codes had been added as described
above, markup had been stripped, and the resulting character sequence had
been passed to an implementation of the Unicode bidirectional algorithm for
plain text that produced the same line-breaks as the styled text."

I'm not entirely sure what the expected output of that would be, but I don't
think it would be the same as any of the above results. So I think the CSS
spec may contradict itself here.

[Question 6 (CSS)]
I put a small test page {4} together and noticed that when I apply the the
CSS rules to essentially the same code above, using a class attribute in
XHTML 1.0, the block is automatically right-aligned in Firefox and IE.  I
don't think this is appropriate behaviour.  As far as I'm aware, there is no
indication in CSS2, CSS2.1 or HTML 4.01 specs about default text alignment
for block elements to which directionality is assigned.  I suspect that this
may show that there's a need for such rules.

[Question 7]
After all this, my assumption as a user would be that if I apply the
override CSS properties to a block level element, all of the contents of
that block element would be ordered as indicated, not just the inline text,
despite the fact that <bdo> was an inline element in HTML.  Do people agree
with that?


PS: I put together a short test page {4} that takes the code above and does
three things:
[a] applies unicode-bidi: bidi-override; direction: rtl; using a class
attribute to the div
[b] remove the class attribute and add &#x202E; (RLO) immediately after the
opening tag and PDF immediately before the closing tag
[c] same as [b] but with all markup inside the div removed

The results are as follows: 
[a] IE, Firefox and Opera behave as per CSS2 - ie. inline text is reordered,
but text in the block is not.
[b] Firefox and Opera do the same as in [a], but IE (including IE6 and IE7
CR1) do not reorder the line of inline text after the paragraph
[c] IE, Firefox and Opera reorder all the text and display without line

{1} http://www.w3.org/TR/2005/WD-xhtml2-20050527/mod-bidi.html#adef_bidi_dir
{3} http://www.w3.org/TR/CSS21/visuren.html#propdef-unicode-bidi
{4} http://www.w3.org/International/tests/test-rlo-blocks.html

Richard Ishida
Internationalization Lead
W3C (World Wide Web Consortium)

Received on Monday, 18 September 2006 16:14:41 UTC

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