- `.carousel-caption` doesn't need a z-index. There are no non-static element in its containing block (`.carousel-item`)
- The `z-index` of `.carousel-indicators` is way higher than it needs to be (`15`), the highest sibling `z-index` is `1`, so it can be set to `2` to be the top layer.

For Gradient, We have to use the background-image instead of only background.

* Use escape-svg() function

* Update theming.md

Previously deprecated in v4.x, this clears out the now unused Sass option and removes some unused mixins. Arguably we could remove more, but I like the hover-focus mixin and we make extensive use of it across the project.

Firefox currently seems extremely fickle - with `pan-y` if fires pointercancel as soon as a touch strays even a pixel or so vertically.
While `touch-action: pan-y` would be ideal (allowing users to scroll the page even when their finger started the scroll on the carousel), this prevents a swipe that isn't perfectly/only horizontal to be recognised by Firefox.

- my fault, my original advice of using `touch-action: pan-x` is exactly the value we *don't* want to have the browser handle...

By applying the transition: CSS property only to classes that are
active during sliding, we avoid an unnecessary, non-zero-time
animation that although mostly invisible, does interfere with e.g.
z-index based parallax

1. It's == "it is"

2. The subject of the sentence is "Only one" (not "these") so the verb must agree with it.

Feel free to do nothing with this change, use it, etc - I just saw this while reading through the code.

* Move helpful code comments

* Add .carousel-fade option to Sass

* Document example of the fade carousel

* more logical warning of the .active class on carousel items

The option is set to `always-single-line` in stylelint-config-standard which we extend.

* Update the form focus mixin to use a manual `$enable-shadows` check so we can always ensure a focus state for accessibility and consistency

* - Add new `$input-btn-focus-width` and `$input-btn-focus-color` variables.

- Replace separate `$btn-focus-box-shadow` and `$input-focus-box-shadow`
variables with unified `$input-btn-focus-box-shadow` to match our
combined variables approach elsewhere.

* Put new focus width var to use in buttons mixins

* use new button input-box shadow var

* Add a new mixin for quickly adding linear gradient to components when $enable-gradients is set to true

* use correct var

* fix focus shadows in button mixins

* Add opt-in gradients to alerts, buttons, carousel, custom radios and checkboxes, custom file input, and dropdown items

* Generate .bg-gradient- utilities

* add headings to colors page and document bg-gradient utils

* update the button color for active status, check with yiq as it's done for basic state and hover state

Use `display: block` since we're not really using any flex styles on the `.carousel-items` (only their controls and indicators). For the indicators, since moving to `width`, our indicators grow to fill available space, so we need to change `flex: 1 0 auto` to `flex: 0 1 auto` to prevent that.

- Drops the `if-supports-3d-transforms()` mixin since all our browsers do support it

- Updates carousel to not rely on that mixin

* Remove most of custom cursor: pointer; implementations

* turns out that's not so much of a thing

- Drops the absolute positioning of the icons within the left/right controls. We have to keep the controls themselves positioned though since we're overlapping HTML elements here.

- No more position, left, right, or margins involved; just some justify-content and align-items.

- Add some comments for explaining which flex property-value pair does what.

- Remove the unapplied font and line-height stuff now that we're all SVGs and flexbox here.

This revamps the indicators to use flexbox instead of inline-block for added flexbility (hah). Indicators now automatically scale based on the number of elements present, and max out at the `$carousel-indicator-width` instead of always being that wide.

* Add variables for transitions

* Add variables for transitions