[Touche]

The parent didn't quote the spec, they quoted a GitHub issue with 71 participants. If you want to post a link to the spec text please do.

If true I'm not sure it matters if every browser throws an exception.

[joshuacc]

User pfooti's comment referenced the specification[1]. You said that a browsers' implementation of the DOM API proves that it is illegal. I then linked to an issue with a detailed discussion of how the DOM API isn't consistent with itself, much less the HTML specification.

[1]:http://www.w3.org/TR/html-markup/syntax.html#syntax-attribut...