I do hope it has or gets enough support to keep adding content though, as the level of quality is definitely a step above most free tutorials.
If I did have a complaint, it would be that i am unconvinced that using custom elements for layout is a great idea. It might work really well for apps, but isn't what I would think of as terribly semantic.
As for the implementation, it's possible to apply the axiomatic styles to standard HTML (directly, or via React, Vue etc). The examples happen to use web components (custom elements) bc of author preference, I think. I prefer React bc it has a better SSR story and doesn't require JS to be enabled.
As for semantic markup per se (a tangent, given my explanation above), your a11y efforts are waay better spent elsewhere.
also want to add: a good CSS thing to learn right now is the CSS grid. you can hammer out the blocking layout of your pages so quickly with a grid.
This relates to similar thoughts about breakpoints being a sometimes-helpful crutch of sorts, vs their typical adoption as a "standard" set of predefined viewport dimensions designers should target. Because it ignores users who config their browsers to use larger fonts (a larger demographic than all IE users).