Add an error for invalid children of <ul>

[jdufresne]

>>> import html5lib
>>> parser = html5lib.HTMLParser()
>>> parser.parseFragment('<ul><ul><li>item</li></ul></ul>')
>>> print(parser.errors)
[]

I would expect an error like: "Element ul not allowed as child of element ul in this context." This is the error reported by W3.org's validator.

The spec states only <li> tags are valid children of <ul>.

[gsnedders]

We don't implement a validator, and as such we don't implement the content model constraints. The parser only reports parse errors, which are explicitly called out in the Parsing HTML documents section. A quick skim makes it look like that shouldn't produce any parse errors. Implementing an actual validator is a large project to undertake, and really something out of scope here.

[jdufresne]

Ah, I see. Thanks for the response.