HTML tags are declarative. JavaScript is Turing-complete. there's a fundamental difference in the expressive power. something's being Turing-complete means that anything whatsoever that is computable with a theoretical Turing machine is also doable with that something, even if in convoluted ways. not being Turing-complete means there are constraints in what can be computed. being purely declarative is a far more stringent constraint that renders it basically useless for anything that deserves to be called computing, or programming for that matter. there's an entire and fascinating field of computer science called formal languages and automata that studies these differences (and a lot more :-)