Adding Microsoft Edge and the rendering engine EdgeHTML.

Closes #18303
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Closes #18141

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Closes #18314 by merging a tweaked version of it.

Revert "Added button subsection to the component section"

Added button subsection to the component section

Check HTML validity of JS visual tests too

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Docs: Separate Tether out from docs.min.js

We can't include it in docs.min.js because docs.min.js includes application.js,
application.js depends on bootstrap.js,
and the tooltip portion of bootstrap.js depends on Tether.

So instead, we need to load Tether separately before bootstrap.js

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Adapt uglify for the new version.

Revert "Make jQuery 2.0 the minimum supported version."

Reword jQuery version mismatch error message since jQuery v3 isn't supported.

This partially reverts commit 9eded912.

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Docs: touch responsiveness optimization

Admittedly wordy info dump on why we use `touch-action:manipulation`

refs https://github.com/twbs/bootstrap/pull/18051#issuecomment-156375970

Add touch-action:manipulation on interactive elements to avoid 300ms click delay

Block level cards

fixes #18241: bump up border-radius values on .label-pill for when customization means the 1rem value doesn't cut it

Document links with text emphasis classes

Autoprefixer callout in Build tools docs