Grid is a DSL certainly, but it's a very useful DSL to learn. I'd even argue that people skip Flexbox or forget everything they (think they) know about Flexbox and just learn CSS Grid.

One of the first callouts in the article is a suggestion to read the same site's interactive guide to CSS Grid if you aren't familiar with it: https://www.joshwcomeau.com/css/interactive-guide-to-grid/

> I'm not confusing rem with em, em are even worse, but still hard predicting what a rem might be, before arguing we shouldn't I'd like to stress out we should once it's used like that "@media (max-width: 32rem)"

`rem` is "just" the root Em, the em in the root font size (the font size of the `<html>` tag itself). In most browsers, with no `<html style="font-size: X;">` override (or `:root { font-size: X; }` stylesheet) that defaults to 16px.

In the Bootstrap 3/4-era responsive breakpoints you often see something like `@media (max-width: 512px) {}` to mean "mobile width". The reasons to migrate those sort of breakpoints to `rem` units instead include 1) adapts to custom browser settings (a user may set their default font size larger, for instance, for visual acuity reasons, which can be a useful accessibility desire), 2) adapts in some situations to zoom settings (browsers are allowed to style the website at a higher zoom as if the user had chosen an equivalent larger base font size), and 3) in the "retina screen" world where resolutions on a phone screen may be higher than desktop resolutions despite smaller screen size, using "roughly 32 `M` characters wide" is a better intuition than "exactly 512px wide" (and mobile browsers can lie a bit less about their pixel widths to meet your breakpoint assumptions).

You can back-of-the-envelope math `X rem * 16 px/rem` to get a pixel approximation if you need one, and that estimate will hold in a lot of cases in a majority of browsers, but leaves the browser more flexibility in adapting to adapting to user desires and hardware quirks than raw pixel counts.

New layouts with CSS Subgrid