I started out writing plain HTML and CSS for small businesses that needed a shopfront on the web. Those clients paid by the page and read it on patchy mobile data, which taught me to care about page weight long before it became fashionable.
Today I reach for Bootstrap to handle the grid and the components, then add only the CSS a design genuinely needs on top. I would rather ship four pages that work properly than twelve that almost do.
On Saturdays I mentor at a local code club, where explaining the box model to beginners keeps forcing me to understand it better myself.