Implement more helpful error message that is caused by <foo unquoted=/>

[zcorpan]

See whatwg/html#2665

In particular for foreign content, when an end tag is missing and the start tag had / as the last character in of an unquoted attribute value before the >, a probable cause of the error is that self-closing tag was intended.

[j9t]

@zcorpan, can you advise what we should change exactly?

The only scenario that looks incorrect when we do throw an error (that we can make more helpful) seems to be something like this:

<!doctype html>
<title>Test</title>
<foo-bar unquoted=/>

But even in a case like this, we throw “Unclosed element foo-bar.” It doesn’t specifically call out the slash but seems to point in the right direction (element is not closed).

I may be missing something and perhaps when we closed whatwg/html#2665, this became moot, too, but happy to look into this if there’s something specific to adjust.

[zcorpan]

A more likely scenario is when the attribute value is something more than a single / and the element supports self-closing (i.e. is an SVG or MathML element) as in <svg><circle r=1 class=foo/></svg>.