[mattwad]

That's not entirely true. Even if you use something as full-featured as Material UI, there's still opinions on how elements are composed, when to use font colors and sizes, etc. Without documenting what to do in common cases, every page will end up looking slightly different. I'm the only dev on my team but I already started a styleguide to help me see how CSS changes affect all components, and as a way to copy/paste common patterns.