Don't take this personally, but I feel like your name might not actually be Mark Otto...

Right on! Let me know what you think of v4 once you dig into it :).

Well written and presented, but we do have many of these mixins and the Sass map in our source code with v4 :). You can loop over $grid-breakpoints and modify, add, or remove map values. You can use the breakpoint-up, breakpoint-down, breakpoint-between, breakpoint-next, etc to craft some crazy cool responsive CSS with our provided mixins.

Check out the v4 source at https://github.com/twbs/bootstrap/blob/v4-dev/scss/mixins/_grid-framework.scss.

Hey everyone, holler if you have feedback or questions. I'm pretty excited to finally have this out and stable!

Holy crap, this is awesome Seth! Well done.

We'll very likely be bringing it back. I underestimated the appeal and usage of it when I removed them :).

No, I'm afraid not—components are sized in rems. Some ems from time to time. However, breakpoints are actually still in pixels.

The .hidden classes presented an issue where we would need their own variants for every single display value—inline, inline-flex, inline-block, block, flex, table, etc. And, we'd need that in two directions—up and down.

With the new approach, you have the same singular set of classes that are more descriptive given their CSS property and value names. For example: d-none d-lg-flex vs d-flex hidden-lg-down. The former is a single direction of mobile-first classes working together, whereas the latter is two directions and two class names.

Hope that helps explain things a bit!

Nice, that looks great! :D

Y'all are the best, thanks for the kind words. Please don't hesitate to holler with feedback!