you'd understand even better if you separated presentation (CSS) from content (HTML) and kicked the mind-numbingly dumbass TRASH that is bootcrap to the curb; what with it's broken nonsensical use of presentational classes.
HTML is for saying what things are, grammatically and semantically. If you are choosing ANY of your tags, classes, or ID's based on what you want things to look like, you've failed to divine the purpose of HTML. Just as if you are using your CSS and/or markup JUST for what you want it to look like on screen, you've failed to divine CSS' purpose.
It is to this end the ONLY thing you can learn from HTML/CSS frameworks like Bootcrap or Failwind is how NOT to build websites.
In all this industry has too much of an obsession with "appearance first" and it ends up telling large swaths of users to sod off. But how dare those of us as users who understand the development side of things speak out against all this predatory framework BS.
It's part and parcel of why we have so many artists under the DELUSION that they're some sort of "designer". News flash, those who don't understand semantic markup, non-visual UA's, separation of concerns, usability, accessibility, and the dozens of other engineering concepts that make up a quality design? They're not designers, they're pixel pushing artists slopping out pretty pictures that more often than not tell the end user to go **** themselves.
Just as the fools calling themselves "developers" who use idiocy like Bootcrap aren't qualified to write a single blasted line of HTML or CSS.
More so when such derpitude suckers you into using JavaScript for things that haven't needed scripting in over a decade to accomplish. Like those accordions of your FAQ section slopping out dozens of div and endless idiotic classes for what today should be <summary> and <details>... and has no scripting off graceful degradation when we've not needed JS even prior to summary/details thanks to either :target or :checked.
The ONLY thing bootcrap is teaching you is how not to use web technologies. The same could be said of the fact you consider "figma" a design tool instead of just more bass-ackward "appearance before design" artsy-fartsy nonsense. Again, perpetuating the myth that design is art unto and of itself.
Because you're RIGHT, you just haven't discovered just how right yet. Understanding HTML and CSS won't just make you a better designer, it makes you a designer. If you don't know HTML, CSS, accessibility, and when to keep it in your pants with goofy scripted animated trash, you're not a designer at all.