Shadow DOM 201: Update CSS Variables syntax to the latest one.

The syntax for custom properties changed from `var-foo` to `--foo` in the latest
spec. This updates the syntax used in examples. #1133
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Remove the mixins for CSS variables.
Replace `font` with `font-family`.

The `font` shorthand requires at least `font-size` and `font-family` to be specified.
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Fix :host css
Fix :host css
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Update Web Audio post to reflect prefix removal

Chromium 35 unprefixed `AudioContext`. Since both Chrome 35 and Opera 22 are stable now, now’s a good time to update this post accordingly.
Merge pull request #1136 from mathiasbynens/patch-1

Update Web Audio post to reflect prefix removal
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Merge pull request #1134 from myakura/shadowdom-201/cssvariables

Shadow DOM 201: Update CSS Variables syntax to the latest one.
Merge pull request #1137 from html5rocks/master

Merging to live lots of changes and fixes