A powerful programming language here could be assembly or Lisp--that hardly makes it a good tool to use to do the things we expect for making user interfaces.

Simple declarative DSLs (such as CSS) are head-and-shoulders above a general-purpose programming language, especially in a situation where logic is not really needed or required. CSS, fugly though it can be, very neatly and thoroughly solves the issue of "How do I style this content?" without adding the complexity of a programming language.

Not all things want to be programmable and flexible.