Unfortunately I feel like there are too many different philosophies behind web design right now to point you to any given site. Mostly because any popular and complex site is going to use some levels of abstraction, i.e. React or Elm or similar. And so a lot of well-programmed websites don't have readable user-facing code. Or, on the other side of the spectrum, there are plenty of sites that value simplicity to the point that they have no-JS, minimal CSS, and super readable HTML. But that's not exactly hard to do.
One of the ways that I learn is by picking out a given HTML tag, for example, from the soup of a website's source code. And then reading the MDN page for that tag, which has information on the idiomatic ways to use it.
One of the ways that I learn is by picking out a given HTML tag, for example, from the soup of a website's source code. And then reading the MDN page for that tag, which has information on the idiomatic ways to use it.