[rty32]

It is not a choice in many situations. For a large company that has many different teams that own many different parts of a product, even though teams adhere to the same "UI standards", things get complicated quickly. For example, CSS classes that have name conflict can cause UI to break (which happens more often than you think, and strictly adhering to naming rules is just hard for humans). That's just one example. Custom elements with shadow DOM is a simple and straightforward solution to this, and it makes sense -- JavaScript code are scoped to modules and use imports/exports to define interfaces, and it is just natural to do that for UI instead of putting every class that other people don't care about in the global CSS space.