Fix minor HTML tag mismatches

Fix minor HTML tag mismatches

Fix some HTML tag mismatches and other glitches. These are minor, and don't appear to cause browsers any problems, but for the sake of correctness:
* A closing td which should be a closing div.
* A closing h3 for a corresponding h6
* Two tables with `<thead>` tag and missing `<tr>` around the `<th>` headings.

test change for thead fix