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These are available in _type.scss but not documented.  These classes are useful for styling without adding semantics.

Refs #18458
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It has been removed from the HTML specification: https://github.com/whatwg/html/issues/336
Only Presto Opera ever implemented a special UI for it.

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also, stray "blocks" in the "Sample output", which doesn't seem to
belong there.

use `<code>` (as it's still semantically code that we're marking up) and
make the example actually multiple lines...

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Previously, this read "Bootstrap doesn’t apply this automatically as it causes complications to other image formats." 

In this sentence, the number of the subject and the verb don't agree, and the word "this" is unclear.

These can be replaced by their `.text-xs-*` parallels.
This also avoids any complications from interactions between the responsive and non-responsive classes.
(e.g. `<div class="text-left text-md-right">`)

Refs #18300
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[ci skip]

Admittedly wordy info dump on why we use `touch-action:manipulation`

refs https://github.com/twbs/bootstrap/pull/18051#issuecomment-156375970

Refactoring according the comments on https://github.com/twbs/bootstrap/pull/18104.
Closes #18131

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Fix #17888

Docs should indicate that `_reboot.scss` uses the variables, but to see their values users would look in `_variables.scss`.

[ci skip]

Fix #17166

/cc @cvrebert

Mostly links.
Fixes #17056.

Example:
> ... SCSS variables (e.g., `@enable-gradients: true`) ...
Referenced SCSS but used the LESS syntax. Changed to:
> ... SCSS variables (e.g., `$enable-gradients: true`) ...

Closes #17114