For me those rules are not the fundamentals. There's more you need to know to actually get CSS to work (and I still usually have to guess and google and try random things to get it to work because my knowledge is lacking)

In particular trying to get content of some flexbox to fill the space provided by the flexbox itself, especially in a single non-scrolling app like page where you're trying to constrain everything to fitting in the window. Like the rule that you have to set min-width to 0 or something to get the desired behavior.

https://makandracards.com/makandra/66994-css-flex-and-min-wi...

That's not the only gotcha

I suppose you could argue that's not fundamental but to me, basic layout that most standard native UI toolkits would provide is an essential feature and yet to achieve it in CSS requires knowing a few obscure tricks

I wish someone would write some other UI language that would translate into CSS so I could write

    full window frame
      left side
      right side
        toolbar
        content

And it would generate the correct CSS to make it work

To put it another way, imagine you want to repo the Keynote UI or Numbers UI in a webpage. Those are basic app UIs and yet they just slightly less than straight forward in CSS. I'm not saying they are hard, only that most people will have to goole some non-intuitive tricks to get things to work which is a different experience than trying to do them in a native UI kit where it's more likely fairly clear how to achieve it.