require void elements to be lowercase

[Rich-Harris]

Fixes #137. Not entirely uncontroversial, since <IMG> and <LINK> etc are valid HTML, but the alternative is quite tricky (we'd essentially need to parse twice, which is a lot of extra work to handle a real edge case).

Any thoughts?

[evs-chris]

👍 Only heathens use upper-case tag names, leave their elements open, and don't quote attributes. 😆 Seriously though, if you want to be super accommodating, you could leave it case sensitive and throw in the all-upper-case variations too.

[Rich-Harris]

if you want to be super accommodating

Good point, that would be doable. Going to put it in the nice-to-have pile for now though 😀

[Andorbal]

I like requiring void elements to be lowercase. It may catch someone of guard the very first time they try to use IMG, but as long as the error says what's wrong, I think the consistency is worth it.