* SVG accessibility fixes
- add `focusable="false"` to all SVGs, to stop IE from including the `<svg>` in its default focus cycle
- completely hide decorative SVGs using `aria-hidden="true"`
- add `role="img"` to meaningful/"content" SVGs, ensure they have an appropriate `<title>` and/or `aria-label`
- simplify placeholder default title
* Simplify usage of placeholder.svg in the docs
- in most cases, no need for separate/redundant text and title (now that we make text itself part of the alternative text explicitly)
- no need (expect in rare cirumstances) to describe the placeholder image (unless the look of the image is important/the actual content, as is the case with the image thumbnail class)
* Simplify example.html rewriting of <svg> to <img> code sample
- as the `alt` for the placeholder isn't really important, just add an ellipsis instead (otherwise, to be correct, we'd have to construct the whole "title+text" construct like we do now in the
placeholder.svg itself
* Change svgo configuration to retain role attribute and add/force focusable=false
- regarding `focusable=false`, see https://github.com/svg/svgo/issues/1011 (which would be the "proper" SVGO fix) and the (hacky) solution https://github.com/svg/svgo/pull/817
Edge/Mobile appears to have moved support for `-ms-viewport` behind a flag at some point, instead supporting meta viewport directives.
Removing this altogether also fixes issues in IE11 on high-dpi displays - see https://github.com/twbs/bootstrap/issues/21780
The default font weight value for <strong> <br> in reboot was
set by a fixed value we now use the variable $font-weight-bolder
to let users override this value.
Callouts are already excluded from ToC, but due to a limitation in jekyll-toc they are still being included.
We should revisit this if the bug is fixed later.
