What do you need to read? The source of web sites that have good design. Use developer mode on a browser. You can edit their CSS live and see the results. You

What do you need to read? The source of web sites that have good design. Use developer mode on a browser. You can edit their CSS live and see the results. You can change the viewport and see how it renders on mobile. You can figure out what's using JS and you'll come to your own conclusions on whether that's the right way to do things. While you're reading, start building things from scratch. MDN is a great resource if something doesn't make sense.