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Re: Multiple Background Images

From: Ben Ward <benmward@gmail.com>
Date: Mon, 29 Nov 2004 21:29:05 +0000
Message-ID: <ef5d0f2f04112913292fe8eaed@mail.gmail.com>
To: dolphinling <dolphinling@myrealbox.com>, www-style@w3.org

OK - having pondered for a few days, how's this for a proposed
specification (not in 'proper' specification language, I'm afraid. I
don't know how to do that right, so I'll just try and make it clear.
If this solution is viable and accepted, then I imagine more competent
people than myself on the WG can word it as appropriate ;-).).

>>>
Multiple backgrounds are represented using an indexed property
(background(0), background(1) ... background(n)). This indexing
applies to all background properties (background-xxx and the generic
background property as well).

The index should be sequential, but bears no impact on the layering of
backgrounds. Instead, a User Agent should layer backgrounds in the
order in which they are declared in the CSS, lowest first, unless
overridden by the background-z-index property.

"background-z-index" declares the 'height' from the page that the
background image is displayed, relative to other background images for
this element. This value has no relationship with any other
background-z-index or z-index value of any other element.
The value can be any integer value in [a sensible range]. If no
background-z-index property is declared then the value is calculated
by incrementing the previously declared background's
background-z-index property by 1. If no background-z-index is
specified for the first declared background of the element then a
value of '0' is assigned.
>>>

One conflict that would need resolution: 

In 'background' shorthand, the background-z-index may be declared
alongside all other properties, but conflicts must be averted with
unitless background-position properties. Otherwise, you end up with
something like this:

	background: url("blah.png") 50 50 3;

50 50 could be the background-position with a z-index of 3, or it
could be 50 3 for position while 50 as z-index.
This is further confused if you specified a single value for position:

	background: url("foo.jpg") 50 2;

It is only necessary to specify one background-position value, but
this could be 1 + z-index or just a normal 2 value.

I can't find in the spec [1] whether it is mandatory to include units
with position, so assuming that you are permitted to, then I have two
suggested solutions:

1) It is specified that background-z-index must be declared AFTER
background-position, and that when background-z-index is declared,
both background-position values must be provided. This rule should
allow user agents to differentiate the properties in the shorthand as
there will always be three values, with background-z-index as the last
integer value (where background-position values are not given units).

2) The spec should strictly declare that lengths for
background-position must be specified with units always, even 0. This
way, any integer in the shorthand without a unit specified /must/ be
background-z-index.

I'm inclined to go with the second, since that is far clearer cut than #1.

Regards,

Ben

References: 
[1] http://www.w3.org/TR/REC-CSS2/colors.html#propdef-background-position
Received on Monday, 29 November 2004 21:29:07 GMT

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