* Get this party started by removing mention of Normalize.css

* Nuke the old comment, consolidate to a single line and number as appropriate

* Bring over styles for HTML element from Normalize to Reboot

* Move margin override for body element from Normalize to Reboot

* Drop the block reset for HTML5 elements in IE9- from Normalize given we dropped IE9 support

* Building on previous commit, do the same thing for figure, figcaption, and main

* Remove IE9- display from Normalize given our browser support

* Drop IE8 figure margin because we're IE10+

* No need for the h1 overrides because we reset these font and margin styles anyway in _type.scss already

* Drop Safari 6 b and strong normalization because we're Safari 8+

* Remove mark styles for IE9- from Normalize

* Remove old iOS audio fixes from Normalize

* Remove IE9- display for progress from Normalize

* Remove more IE9- rules from Normalize

* One more IE9- display removal for canvas element

* Move pre overrides from Normalize to Reboot

* Move over some link resets to Reboot, drop others

- Move over background-color and text-decoration
- Drop focus outline change given it affects the offset on hover of
focused links

* Move over more code element resets, consolidate with pre overrides, too

* Move over sub and sup wholesale

* Move over img normalization to Reboot

* Move over SVG override too

* - Drop dupe hidden, but add comment for it
- Move over template
- Move over summary

* Remove bulk of @viewport comment

* edit down that code comment

* consolidate html-based normalizations

* update comments

* Consolidate abbr styles

* move over more type elements

* move over hr changes

* move over form controls and more

* move over button resets

* move over firefox button changes

* move over search changes and more

* we nuke all these styles for fieldsets anyway, so outright remove them

* no need for those, we override them

* move over legend, fieldset, progress

* line break

* delete normalize file

* linting

* update comment

* clarify docs mentions of normalize and reboot

* remove normalize excludes from linter

* remove normalize excludes from cli task

* linting

* callout license since we forked part of normalize

* Improve comments, move table background reset to .table class instead of in reboot

* trailing space

* Update ie10-viewport-bug-workaround.js year

* Update narrow-jumbotron copyright year to 2017

* Update carousel/index.html copyright year to 2017

* Update browsers-devices.md copyright year to 2017

* Update change-version.js copyright year to 2017

* Remove IE compatibility mode meta tag from docs, examples, and JS tests as we no longer support IE9 and IE8
* update and remove some IE bits from our supported browser page
* update introduction.md to match
* reword starter template intro

* remove the $enable-flex variable option

* remove bootstrap-flex.css dist file and it's grunt task

* remove the separate flex css file for docs; it's all the same now

* remove flexbox docs (porting some to the main grid docs in next commit)

* clean up few grid docs bits to simplify copy, start to mention flexbox

* port relevant flexbox-grid.md content to grid.md

- clean up mixins
- update how it works section
- bring over sizing and alignment sections

* remove the $enable-flex from the options.md page

* update lead paragraph to mention flexbox

* update migration to mention loss of ie9 support

* remove mention of flexbox dist file

* clarify IE support

* making a note

* remove flexbox variant mentions from component docs

- updates docs for media object, navs, list group, and cards to consolidate docs
- no more need to callout flexbox variants since it's now the default

* remove $enable-flex if/else from sass files

* remove flex dist files

* update scss lint property order to account for flex properties

* linting

* change to numberless classes for autosizing, wrap in highlighting div

* bump gruntfile and postcss to ie10

* redo intro sections

* rearrange

* phew, redo hella grid docs

- rearrange all the things
- consolidate some bits

* remove reference to flexbox mode

* more border action for demo

* Make some changes to the .card's in .card-deck's to ensure footers align to the bottom

fixes #21072: clarify location of variables file (and avoid linking to a file that will be broken depending on the branch name)

* descriptions for getting started pages

* descriptions for layout

* add content page descriptions

* more descriptions, updates to some existing ones

* correct site url

* add social stuff to config for twitter cards

* add twitter meta tags; use large image for homepage and regular card for all others

* add the assets

* more site config

* more social shiz to partial, remove existing meta for the partial, remove page title from homepage for simpler if statements

Refs https://bugs.chromium.org/p/chromium/issues/detail?id=454172#c22

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Closes #20165

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Refs #19990
Continues the degruntification process.

Also removes mq4-hover-shim for now,
since it doesn't yet implement the standard PostCSS plugin interface.

Refs https://bugs.chromium.org/p/chromium/issues/detail?id=273306#c34

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️️️ https://github.com/twbs/bootstrap/graphs/contributors

Refs #14839, #19088
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