[CSS] will-change的属性值

1. 语法：

will-change:  auto | <animateable-feature>

其中<animateable-feature> = scroll-position | contents | <custom-ident>

auto：默认，不优化
scroll-position：优化滚动效果
contents：优化内容的改变

<custom-ident>：优化给定名字的属性值变化
如果指定的属性名是缩写，就优化完整形式的所有属性
该名字不能是这些值，unset, initial, inherit, will-change, auto, scroll-position, contents
规范中并没有指定特定值的行为

2. Chrome的实现如下：(截止到2015.8.17)

（1）will-change values that trigger a compositing layer in Chrome:

opacity
transform, -webkit-transform
top, left, bottom, right

source: ComputedStyle::hasWillChangeCompositingHint()

（2）will-change values that trigger a stacking context in chrome

opacity
transform, -webkit-transform
transform-style, -webkit-transform-style
perspective, -webkit-perspective
-webkit-mask, -webkit-mask-box-image, -webkit-clip-path, -webkit-box-reflect, -webkit-filter
z-index
position

source: hasWillChangeThatCreatesStackingContext

（3）also
scroll-position does nothing in Chrome currently.
contents appears to be a compositing layer hint for whatever children are being changed.

3. 参考：

https://developer.mozilla.org/en-US/docs/Web/CSS/will-change
https://github.com/operasoftware/devopera/pull/330