Fix grammar of comment for <label> in Reboot

Add 3 more font-weight variables

Refs #14368

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Ports #18445 to v4.

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Add Wall of Browser Bugs entry for #15990

https://bugzilla.mozilla.org/show_bug.cgi?id=1139853
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Mention Mozilla Firefox meta-bug on Wall of Browser Bugs

https://bugzilla.mozilla.org/show_bug.cgi?id=1230801

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Use !default on $form-icon-* variables

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V4 dev docs library updates

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Update shrinkwrap

Fix docs/search.json.

* only loop through `html_pages`
* skip pages without a title
* stop double escaping the title; use Jekyll's `jsonify` filter to output valid JSON
* remove `date` since we don't use it

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s/grunt-sed/npm script/

Bump Safari to v8 in Autoprefixer settings

The latest OS X Safari version is 9.0.1
Our prefixing policy dictates that we prefix 1 version back from the latest version.
9.0.1 - 1_major_version = 8.0

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Bump iOS to v8 in Autoprefixer settings

util.js: Add comment explaining ~~ trick

SCSS Lint: explicitly enable PseudoElement check

Fixes the IE 9 error.

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