Investigate why HTMLProofer didn't find broken internal link

[harding]

In #167 (comment), @bitschmidty found a broken internal link (anchor) on a PR that passed tests---but the tests are supposed to find broken links to anchors (and they have on many occasions). My best initial guess is that this is because the brokeness is just a capitalization issue, but I need to investigate when I get a moment and see if I can make the test more strict.

[harding]

Also investigate why it didn't find unclosed header and paragraph tags as fixed here: 62ebd4e