@twbs/css-review I think I'm done with renaming here: please step in to review the CSS part before I continue with the documentation part. There are a few questions in the PR description, BTW :)

One more comment while I'm here—I think our top/bottom classes can stay as-is and left/right can change to start/end. This keeps half our classes intact and reinforces a shared language across horizontal and vertical alignment.

Vertical: top, middle, bottom
Horizontal: start, center, end

Does this get us too far away from logical properties? Is it a weird bastardization? I want to keep these class names approachable.

@mdo yeah I've been thinking the same thing, considering the amount of changes here. The target is RTL only for v5, so logical names should only be a first step for this release.

I'll make a step back, and topics like dropdowns, tooltips and popovers will get simplified. However that'd come to us when we'll switch to logical properties: that might require to work on Popper.js at some point…

Consider this first commit as a v6 preview or something—I'll trash it while rebasing 👍

@mdo I think it's ready now. To sum up:

1. only left and right things were renamed — .ms and .me for margin-left and margin-right being the most shocking changes to me (but way less than my previous attempt)
2. docs have been revamped, I guess wording and phrasing could be better — just let me know :)
3. a new RTL section is introduced in the migration guide — above the alpha2 since I don't think it'd be ready for alpha 2;
   • I used details and summary to sum up things a bit, should I drop it?
4. Popper.s related questions should better be solved in main PR before rename anything here (I think there'll be more JS related topics than CSS ones)
5. RFS is a completely different topic, I'd have to open a PR against RFS directly.

Do not forget that it can be merged in my main RTL branch and reworked later :D

Not sure why and where we need additional mixins? We could use @include rfs($padding-start, padding-left);

@MartijnCuppens You're right, I think there's nothing needed RFS side: we don't use its direction-related mixins in our codebase. So until the feature is asked for in RFS, I guess we're safe 👍

So the latest pain point in #30980 concerns Popper.js.

This specific PR is definetely ready to review :)

I've used Google's Flutter for android development which supports RTL by default.
They use start and end instead of left and right as their standard without changing top and bottom to anything else.