What is the issue with the HTML Standard?

https://html.spec.whatwg.org/multipage/dom.html#custom-data-attribute:~:text=If%20name%20does%20not%20match%20the%20XML%20Name%20production%2C%20throw%20an%20%22InvalidCharacterError%22%20DOMException.

If name does not match the XML Name production, throw an "InvalidCharacterError" DOMException.

We forgot to remove this when doing whatwg/dom#1079 and #7991. Since part of the goal there was to reduce the dependency on XML name validation code, we should also remove this.

I'll investigate what exactly the right fix is soon.

/cc @josepharhar