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Refs #18178

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See the IE9 note in https://developer.mozilla.org/en-US/docs/Web/CSS/%3Ahover#Browser_compatibility
The prereqs to trigger the bug are still present in v4.
/fyi @mdo

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Refs #15947
Somehow c256aca1 got lost during the SCSS translation...

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Refs b6e6a05b
We ought to re-check this for iOS 9.1 at some point...

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Refs #16836
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Fix #18002

/fyi @mdo

They already share the same value (`.25rem`)
Refs #17597
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Reunites the family.
Closes #17791

Removed empty row.

Changes grid and container sizes to `px`, as the
viewport pixel size does not depend on the font size.

The actual em values were inconsistent with the docs,
while the docs were not the same as the comments:

* `sm` breakpoint was 34em (544px) not 480px.
* `lg` container max-width was 60rem (960px), less gutter than `md`.
  Changed to 940px, same as Bootstrap 3.
* `xl` container max-width was 72.25rem which is 1140px not 1156px.
  Changed to 1140px matching the comment but not the docs.

Addresses #17070 and #17388.

They can be rounded later on if $enable-rounded: true.

fixes border-width issues when customizing font-size by using 1px instead of .0625rem as default -width

Added usage of $border-width variable in all _list-group.scss references