- Line height & font family from html were directly overridden by body, so they were redundant thus removed
- The remaining properties are moved to body
- Shim for IE10 tags removed
- hr cleanup
  - Combined styles from reboot & type
  - Changed from borders to background
    - This allows us to keep the color inherited.
    - Opacity can be used to change the intensity
    - Size attribute will not change the height instead of making the hr look like an input field
  - Better variable names
- headings cleanup
  - Moved all element styling to reboot
  - Type will now extend the element styling. This way we will save some redundant code
- small & mark are also moved to reboot. type does not contain any element styling anymore
- font weight removed from small, this did not really made sense

Blockquote footer font size is currently hardcoded to 80%, implying (in a comment) that this would result in the default font size. However, since the blockquote font size itself is variable, this is not necessarily true, and 80% of anything actually results in an arbitrary font size.

80% as a default is still fine.

* Drop -x and -y as they're all the same
- Also move -width to elsewhere in the vars because it makes no sense by spacers.
- Update values of -x and -y across main Sass and docs Sass.
* Update docs to reflect changes; link to spacing utils from options page

- had to rearrange vars to re-use heading line-height
- align values while we're at it

* 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

Websites might use different weights such as medium or thin or use light as default weight instead of normal.
Also referenced: #18605 and #16170.

Originally -font-size was added in v3 to set the font-size of the <small> element. Now that we have newer, global type variables, we can use those instead for these kind of things.

Fixes #20859

Introduce $mark-bg variable so that <mark>'s background-color can be customized independently of $state-warning-bg

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- Using the grid classes, they still need clearfixes since rows aren't injected between them all
- Update docs to fix broken classes

This reverts commit deeb74e3.

Reverts #18038
Fixes #18992
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- Rename -height to -height-base to match other vars
- Drop use of -height across the board and rely on it to be inherited
- Adjust padding of .dropdown-header to account for different line-height of h6 heading element (this needs refactoring for variables and rems also)

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/fyi @mdo

Fix #17166

Fix #17052

- new vars for .lead size and weight (previously had none)
- new var for default margin-bottom on h1-h6 elements (previously had none)
- rearranged vars to combine two typography sections into one in variables.scss
- reassigned border-width var to hr-border-width
- made hr-border more specific as hr-border-color