Fixes #9415

patch highlighting hint

Emphasize git HEAD downloads

add HTML5 validation of docs pages to the build

use `data:` URL of empty invalid PNG: src="data:image/png;base64,"

Make docs HTML more valid

fix ID of <select> in fieldset[disabled] example

fixes #9258: no more float bug with full-width grid columns by removing float on all .col-*-12 classes

undo the grid max-width boundary because it makes offsets kinda borked (you'd have to set twice, for medium and large, in most cases--and that sucks)

social-buttons.html: drop 'px' from height+width attrs per HTML5