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
add HTML5 validation of docs pages to the build
add ARIA roles to Dropdown examples in Components docs
add src attribute to holder.js-powered <img>s so that they validate

use `data:` URL of empty invalid PNG: src="data:image/png;base64,"
add alt text to <img>s in components.html
add alt text to <img>s in javascript.html
Components docs: Dropdowns section: add .sr-only toggle buttons so aria-labelledby validates
Navbar refactor

* Now responsive by default (fits better with mobile-first approach)
* Requires `.nav-header` to group `.navbar-brand` and `.navbar-toggle`
for proper mobile display.
* Changed `.nav-collapse` to `.navbar-collapse`
* Simplified examples in docs
Merge branch '3.0.0-wip' of github.com:twbs/bootstrap into 3.0.0-wip
fixes #9400: add box-sizing to pseudo elements as well
finish fixing #9295: add button group buttons to new input group sizing