Headings docs: use put <hN>s within <td>s instead of <th>s

BS3 commit 81ab8ffae0a89e38c105753fc865882e6345b232
rm unnecessary blank id attribute

BS3 commit 10356f9d67e6d5a1f3440e58c81e054f078501d7
use unique IDs in Forms examples in docs

BS3 commit a13fe0a4a04c5712cb77bed6d508c89c9bcbb18b
drop iframe attributes for CSS styling

BS3 commit f4bb660974699f4336d22385e104347f6bd4dce0
add HTML5 validation of docs pages to the build

BS3 commit bb5be0a2f9b9ed789e79603c106e8d2c7cf62305
add ARIA roles to Dropdown examples in Components docs

BS3 commit 3ec793072072773c2d834d99d7e4307629c18058
add src attribute to holder.js-powered <img>s so that they validate

BS3 commit b33989a97791560f064e723cfff2ec5b4ac65eba
add a blank line
mention fubared state of overflow hidden on body in android and iOS

BS3 commit 54a3e4b780b711660e650d909e0e11875d7ff505
enable collapsing for labels, in addition to badges, when empty. nuke empty space between panel heading and list group. don't bother with a .label font-weight var and let's just use bold because windows hates 500. rebuild + first bit of customizer urls. Drop the font-weight on panel labels since heading elements already get it. Refactor navbars to simplify things. Follow up commit: fix collapse clearing and navbar brand/nav spacing. simplify padding in list groups and margins on badges within. alerts aren't dismissable by default, now requires .alert-dismissable if you need a close button. make it so active list group items aren't dependent on an <a> tag. fixes 9266 manually on account of list group file reorganization. fix 9239 by restoring old ie8 responsive img foo. Drop background-clip and background-size mixins since they have no prefixes anyway. inline forms now require use of .form-group for proper alignment of all form controls. inherit font-size in code when inside pre. Newfangled input and button group sizing. no need for width auto on inline/navbar form controls given inline-block on parent form group. 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). no more float bug with full-width grid columns by removing float on all .col-*-12 classes. fix navbar form alignment of inputs by restoring inline-block. add box-sizing to pseudo elements as well

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