fix duplicate id in HTML
drop iframe attributes for CSS styling

From https://github.com/twbs/bootstrap/commit/f4bb660974699f4336d22385e104347f6bd4dce0
add alt text to images
fill out previously empty img alt texts
import HTML5 validation task from Bootstrap; fixes #282
Merge pull request #291 from twbs/html-validation

add HTML5 validation Grunt task