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Re: [css3-writing-modes] a third option for implementing logical properties

From: John Daggett <jdaggett@mozilla.com>
Date: Thu, 21 Oct 2010 21:04:48 -0700 (PDT)
To: "L. David Baron" <dbaron@dbaron.org>
Cc: www-style@w3.org
Message-ID: <359112373.144686.1287720288530.JavaMail.root@cm-mail03.mozilla.org>
L. David Baron wrote:

> I wanted to raise a third option for how to deal with logical
> properties in addition to the two I cited in
> http://lists.w3.org/Archives/Public/www-style/2010Jun/0003.html .

I think the problem with logical properties is not so much one of
implementation difficulty but that it creates a confusing authoring
model that doesn't completely solve problems related to supporting
both vertical and horizontal layout.

Some elements within vertical layouts will have the *same* orientation
in both vertical and horizontal layouts (e.g. images, captions, form
elements).  Unless *all* directionally dependent properties in CSS 2.1
*and* in various CSS3 modules include directional equivalents (e.g.
border-radius), authors need some way of defining styles that are
dependent on the writing mode of a given element. Authors will also need
this to optimize layouts that support both future browsers with vertical
text support and older browsers without.  With only logical properties
supported authors would need to use some form of browser / CSS
capability sniffing to do this.

Once you have some mechanism for doing this, logical properties are no 
longer a necessity, they become syntactic sugar that simplify the
writing to stylesheets to support both horizontal and vertical layout.
With logical properties you still need two rules to handle fallback
horizontal layout in older browsers:

  p {
      margin-left: 2em;   /* horizontal layout in older browsers */
      margin-start: 2em;  /* vertical or horizontal layout in newer browsers */
  }

Existing basic box model properties (margin/padding/border) are
relatively easy for authors to understand, they know what happens when
they use them in stylesheets, they know what happens when they change
them via Javascript. But if you add logical properties on top of these,
the authoring model gets much more complicated for authors wanting to
design layouts with vertical text.  Authors now have to consider the
interaction between two properties and changes via Javascript will have
implementation-dependent behavior [1].  Plus both physical and logical
properties still need to be defined to handle horizontal layout in older
browsers.

Regards, 

John Daggett

[1] http://lists.w3.org/Archives/Public/www-style/2010Oct/0512.html
Received on Friday, 22 October 2010 04:05:22 GMT

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