Re: [css-transitions] Interpolation of side-relative <position> values

On Friday 2016-02-19 18:28 -0800, Simon Fraser wrote:
> There is some trickiness with interpolation of side-relative <position> values. Consider:
> 
> @keyframes shifty {
> from { background-position: 20px; }
> to { background-position: right 20px; }
> }
> 
> Should that work? To make it work you’d have to know the box size when doing interpolation, or I suppose the UA could turn it into a calc() expression?

In Gecko it works because we internally represent computed values of
background-position as calc() of a length and a percent, since the
three-value and four-value variants can all be reduced to that.

This is consistent with the "Computed value:" line in
https://drafts.csswg.org/css-backgrounds/#the-background-position ,
so I believe it's correct, and that because of that "Computed
value:" line, this should work per spec.

(That said, I suspect I may have written that "Computed value:"
line, so I'm certainly willing to discuss if you think it should
change.)

-David

-- 
𝄞   L. David Baron                         http://dbaron.org/   𝄂
𝄢   Mozilla                          https://www.mozilla.org/   𝄂
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             What I was walling in or walling out,
             And to whom I was like to give offense.
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