[css3-animations] Handling multi-valued animation-timing-function inside a keyframe

When used inside a keyframe, the animation-timing-function defines the progress
of the animation between its keyframe and the next.

As noted by David[1] in the mail referenced by bug 14796, the property does accept 
a list of values. When this occurs inside a keyframe we could either ignore the
animation-declaration or use the first specified value.

Gecko and IE10 use the first value and ignore the rest. WebKit appears to use the last.

Thoughts?

[1] http://lists.w3.org/Archives/Public/www-style/2011Apr/0359.html
[2] https://www.w3.org/Bugs/Public/show_bug.cgi?id=14796