Proposal: will-change property (formerly will-animate)

The discussion appears to have converged in the previous post[1]. Here's a
summarily of what I believe is the current consensus.

For justification of the property please see the initial proposal and
discussion.

The property would accept:
will-change: [auto | initial | inherit | unset] | <animatable-feature> [,
<animatable-feature>]+
<animatable-feature> = scroll-position | contents | <custom-ident>

custom-ident will match current and future CSS properties.
I believe we want custom-ident to explicitly reject the following:
none (we decided to punt on that for now), all

Stacking context behavior:
will-change will induce a stacking context if the non-initial value of any
of the specified <custom-ident> match a CSS property induces a stacking
context.

[1] http://lists.w3.org/Archives/Public/www-style/2013Nov/0414.html