My advice, like with anything, is to learn the fundamentals first. In this case, that's CSS and its box model [0]. You don't need to learn every nook, cranny, and idiosyncrasy of CSS, but enough to know what's going on in higher level abstractions. I would just start with an empty html file and play with layouts devoid of any JS. Beware of outdated html/css "tutorials" online and stick to updated resources like Mozilla's MDN. Once you get the hang out modern layout models like flexbox and grid, I would recommend using a 'functional' css library like tailwind [1].

[0] https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_Box_Mod...

[1] https://tailwindcss.com/