The pseudo element argument in some APIs ( getComputedStyle(element, pseudo) and new KeyframeEffect(target, keyframes, {pseudoElement}) is currently parsed in a way that doesn't match the spec - e.g. it allows pseudo-elements without ":", it doesn't parse whitespace correctly if there is argument, and several other issues. There are many failing tests around this. Recently webkit fixed this to more closely match the spec. This matches the webkit implementation and the spec, unskipping most of the tests. Using a chromestatus entry for this as it's a web-facing behavior change.