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[css4-background] background-position-x and background-position-y or logical directions

From: Florian Rivoal <florianr@opera.com>
Date: Thu, 21 Jun 2012 16:55:45 +0200
To: "www-style@w3.org" <www-style@w3.org>
Message-ID: <op.wf9c270z4p7avi@localhost.localdomain>

IE and Webkit support background-position-x and background-position-y, and
it is being used in the wild (primarily for sprites), so I was considering
specifying so that we could have a solid basis to implementing it.

However, if we do introduce this, it prevents us from introducing
writing-mode sensitive logical position keywords.

With physical directions, there is no problem
  background-position: bottom right;
  background-position-x: right;
  background-position-y: bottom;

But this can't work:
  background-position: start head;
  background-position-x: ??;
  background-position-y: ??;

The only solution I can think of is pretty ugly:
  background-position: bottom right;
  background-position-x: right;
  background-position-y: bottom;
  background-position-inline: auto;
  background-position-cross: auto;

  background-position: start head;
  background-position-x: auto;
  background-position-y: auto;
  background-position-inline: start;
  background-position-cross: head;

This would need a precedence rule to deal with all the longhands being
manually set to non auto values.

If we don't care for this (ugly) workaround, I am afraid we have to
decide between logical keywords and *-x / *-y.

There are arguments in both directions.

In favor of -x and -y:
* they are already supported by 2 out of the 4 main engines, and have
   been for many years, so maybe we should pave the cow path and boost
   interoperability with existing content.

Against -x and -y:
* they are blocking another useful extension of background-position
   unless we resort to ugly workarounds
* They can be simulated with variables, so we can live without them:

For instance, here is a typical example of how they are used:
  .icon { background: url(sprite.png) 0 0 no-repeat; }
  #a { background-position-y: 0; }
  #b { background-position-y: -30px; }
  #c { background-position-y: -60px; }
  .icon:hover { background-position-x: -30px; }
  .icon:active { background-position-x: -60px; }

This can be rewritten with variables:
  .icon { background: url(sprite.png) var(bg-x, 0) var(bg-y, 0) no-repeat;}
  #a { var-bg-y: 0; }
  #b { var-bg-y: -30px; }
  #c { var-bg-y: -60px; }
  .icon:hover { var-bg-x: -30px; }
  .icon:active { var-bg-x: -60px; }

  - Florian
Received on Thursday, 21 June 2012 14:56:21 UTC

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