Add aliases for ::placeholder

[ghost]

Should ::-webkit-input-placeholder be added to the spec?

[zcorpan]

...and how should aliasing work for pseudo-elements. (I think serializing should give the unprefixed name. getComputedStyle() is not relevant per spec since it only supports ::before and ::after.)

[zcorpan]

Hmm getComputedStyle() might be relevant after all, https://github.com/w3c/csswg-drafts/issues/717

[karlcow]

SeeAlso https://bugzilla.mozilla.org/show_bug.cgi?id=1424106