[css-animations-1] Undefined procedures to get/set CSSOM attributes

[cdoublev]

The procedures to get/set CSSKeyframesRule.name and CSSKeyframeRule.keyText are loosely worded:

interface CSSKeyframesRule : CSSRule {
  attribute CSSOMString name;
  // ...
};

name, of type CSSOMString

This attribute is the name of the keyframes, used by the animation-name property.

https://drafts.csswg.org/css-animations-1/#interface-csskeyframesrule-idl

Related: #2435

interface CSSKeyframeRule : CSSRule {
  attribute CSSOMString keyText;
  // ...
};

keyText, of type CSSOMString

This attribute represents the keyframe selector as a comma-separated list of percentage values. The from and to keywords map to 0% and 100%, respectively.

If keyText is updated with an invalid keyframe selector, a SyntaxError exception must be thrown and the value of keyText must remain unchanged.

https://drafts.csswg.org/css-animations-1/#interface-csskeyframerule-idl

[cdoublev]

Related: #9363.

the CSS parsing restrictions aren't really relevant here, since we're not in CSS. Being able to make a counter called "1st" should be easily doable in JS, even if you have to escape it in your stylesheet.