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background-position and -origin

From: Eric A. Meyer <eric@meyerweb.com>
Date: Thu, 14 Aug 2008 14:43:01 -0400
Message-Id: <a06240808c4ca29b4c376@[192.168.1.196]>
To: www-style@w3.org

    I noticed the CSS WG tweet about a proposed change to 
'background-position' that would extend it to help define the origin 
corner of a tiled background image.  Since I've occasionally agitated 
for this ability, and have bounced around a few ideas along those 
lines, I thought I'd bring up the topic here for more in-depth 
discussion.
    I find the ability to declare 'bottom 1em right 33%' to be very 
interesting, and it definitely meets the need.  There's a smallish 
part of me, though, that feels like this is mixing two concepts: 
origin corner and offset.  I do think there's a strong case to be 
made for mixing the two, since they're so closely related.
    Another possible solution with its own strengths is to leave 
'background-position' at just declaring offsets, and redefine 
'background-origin' so that it takes three keywords: two defining the 
corner, and one defining the "origin edge".  So to get the origin 
point to be the top right padding edge, you would declare either of 
the following:

    background-origin: top right padding;
    background-origin: padding top right;

That makes the value syntax something like (I left out the "in any 
order" magic symbols for clarity):

    [ [ top | bottom ] [ right | left ] ]? [ content | padding | border ]?

...and then do the comma voodoo to extend that to account for 
multiple background images.  I guess that means defining the above 
pattern (or a cleaner version of same) to be 'bg-origin' and then 
defining the value for 'background-origin' to be '<bg-origin> [, 
<bg-origin>]*'.  The default for 'bg-origin' would be 'top left 
padding' to reflect historical behavior.
    In order to disambiguate things ever so slightly, we could change 
'content | padding | border' to 'content-edge | padding-edge | 
border-edge'.  I'm not sure doing so really buys us anything worth 
obtaining, though.
    I had been thinking that a reason to take this approach would be 
to prevent backwards-compatibility problems, since 
'background-position' with keywords mixed in will be ignored by old 
UAs, but then I realized that any page that uses more than one 
background image (thus invoking the comma-separated syntax) or that 
uses 'background-origin' to change the origin of even a single 
background image is going to confuse old UAs anyway.  So the 
backward-compatibility argument has pretty much the same bearing 
either way.
    At this point I'd be just as happy with either one, but thought 
providing an alternate idea as a starting point for discussion might 
be helpful.

-- 
Eric A. Meyer (eric@meyerweb.com)     http://meyerweb.com/
Received on Thursday, 14 August 2008 18:43:46 GMT

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