add alt text to carousel <img>s
add alt text to thumbnail <img>s
add alt text to media object <img>s
add alt text to <img>s in Images section of CSS docs
Headings docs: use put <hN>s within <td>s instead of <th>s
rm unnecessary blank id attribute
use unique IDs in Forms examples in docs
give navbar forms non-empty `action`s
drop iframe attributes for CSS styling
Merge pull request #9399 from twbs/make-docs-html-more-valid

Make docs HTML more valid