[css-animations] Allow CSSKeyframesRule.name to set reserved values

[Manishearth]

https://drafts.csswg.org/css-animations/#dom-csskeyframesrule-name

Setting this property to a value matching a CSS-wide keyword or any keyword defined for the animation-name property will throw a SyntaxError exception.

However, it is already possible to create a keyframes rule with a name matching a CSS-wide keyword using a string -- @keyframes "initial" {..} works (See https://drafts.csswg.org/css-animations/#typedef-keyframes-name and Example 2 below it). The DOM setter should be able to create such rules, so we should remove the ability for the setter to throw a syntax error and allow all strings.

cc @canaltinova @dbaron

[SimonSapin]

I’m changing Servo’s implementation to not throw, and instead store a value that serializes as a quoted string.

[birtles]

I also checked and noticed that Gecko already allows quoted strings and does not throw when setting reserved values: https://codepen.io/birtles/pen/xdRjap

[dbaron]

Gecko changed to allow strings in bug 1320474, which cites #118.