Does anyone have a good way to approach this? Or for HTML in particular, is there some way to keep up with it (on a decade time scale, not this year's framework hype)?
Its author has been in the web dev field for decades and has written multiple books on the topic. She has got some chapters written by experts in respective fields (CSS by Eric Meyer and so on)
As much as W3Schools is hated, I find their HTML/CSS/JS guides to be a lot simpler to understand than MDN: https://www.w3schools.com/html/default.asp
I am sure there are a lot of good stuff there, but quite a lot has happened with html/css and browser support since late 2011.
For CSS I recommend Wes Bos tutorials about flexbox and grid - https://wesbos.com/courses and this blog: https://ishadeed.com/articles/
Also read MDN often.