Ignore ESLint error in ie-emulation-modes-warning.js.

Although the error is valid, enough time has been spent on old IE's problems, so just ignore it.

Remove https://bugzilla.mozilla.org/show_bug.cgi?id=1282363
because CSSWG deemed Firefox's behavior to be the correct behavior per
https://github.com/w3c/csswg-drafts/issues/314

Replace it with a Chrome bug about changing Chrome to follow the spec & Firefox:
https://bugs.chromium.org/p/chromium/issues/detail?id=269061

* https://developer.microsoft.com/en-us/microsoft-edge/platform/issues/817822/
* https://bugs.chromium.org/p/chromium/issues/detail?id=465274
* https://bugs.chromium.org/p/chromium/issues/detail?id=510079
* https://bugs.webkit.org/show_bug.cgi?id=158276

The transitioncancel event was shipped in Firefox 53 
https://bugzilla.mozilla.org/show_bug.cgi?id=1264125

(#22414) Rename for consistency `$custom-checkbox-border-radius`, `$custom-checkbox-icon-checked`, `$custom-checkbox-indeterminate-bg`, `$custom-checkbox-icon-indeterminate`, `$custom-checkbox-indeterminate-box-shadow`, `$custom-radio-border-radius`, `$custom-radio-icon-checked` to `$custom-checkbox-indicator-border-radius`, `$custom-checkbox-indicator-icon-checked`, `$custom-checkbox-indicator-indeterminate-bg`, `$custom-checkbox-indicator-icon-indeterminate`, `$custom-checkbox-indicator-indeterminate-box-shadow`, `$custom-radio-indicator-border-radius`, `$custom-radio-indicator-icon-checked`, respectively

fix location of favicons so it loads when viewing the examples. This is the only one I found in the project, so I am assuming it is the right one to point to (#22895)

/cc #23070

Bootstrap itself doesn't use any `::first-letter` or `::first-line`
styles. These selectors also cause(d) problems in IE11 (see
https://github.com/h5bp/html5-boilerplate/pull/1799) and currently cause
a nasty rendering bug in Chrome where the first letter is vertically
shifted (see https://github.com/twbs/bootstrap/issues/21771)

As h5bp has now removed these as well, no need to keep and comment them
out. See https://github.com/h5bp/html5-boilerplate/issues/1961 /
https://github.com/h5bp/html5-boilerplate/pull/1962

move the about directory into the docs to avoid collision with the multiple versions of our hosted docs

When focused, these buttons styled as links still give good visible focus indication through the underline, so this should not be a concern. Closes #22655

also remove the overflow-x. when you specify either overflow-y or overflow-x, the other is essentially nullified and thus ineffective anyway

fixes #22977