Remove --bs-headings-color CSS variable due to backward compatibility issues

[mdo]

Fixes #36365.

The null value breaks existing styling where hyperlinks with a heading helper class no longer appear blue. This will have to be resolved most likely in v6 I'd think to avoid any other BC issues.

[ffoodd]

Cannot check right now but couldn't a fallback value be a better fix? Something like var(--bs-heading-color, inherit) — to be tested, could be currentcolor or explicitly using body color too.

[mdo]

Any fallback like that presents the same issue unfortunately—links with .h1 would appear dark gray instead of Bootstrap blue as it'd be inheriting from the body.

[mdo]

@ffoodd @julien-deramond Thoughts on maybe adding a new --bs-heading-link-color and adding some new a.h1, a.h2, etc rules? As of right now, any fallback keyword value like inherit, currentcolor, initial, etc would result in the same behavior. The intent is to preserve the blue link colors on heading utilities (and not, fwiw, on children elements like h1 a to be clear). This might still then be a breaking change, but it preserves a new CSS variable and adds even more customization later (e.g., the ability to make heading links custom from body paragraph links which lots of sites do).

[ffoodd]

Usually not in favor of using element and class selectors together, but not sure to have a better solution.

There're at least two ways I can think of:

• using the :any-link pseudo-class which would feel a bit more elegant than element selector—but wouldn't differ that much;
• using a custom cascade based on custom properties, e.g. var(--bs-link-color, var(--bs-heading-color)) which would ensure the first get applied (props to Miriam Suzanne for the technique).

I can't find any other way around for now. Any thought?

[mdo]

Hmm... for the second option, both values are always set (globally), so not sure a custom cascade would help much?

[ffoodd]

Uh, you're right. I don't think it's safe to move --bs-link-color to links reboot, sadly, since it could be used on other elements than links.

I can't find any other way. Let's go for a.hn then?

[mdo]

For now, going to outright remove this one. We can circle back on a new variable when we're able.