A big part of React (and arguably the whole point of React), is the way in which state changes are handled and re-rendered. A React version of this progress bar

A big part of React (and arguably the whole point of React), is the way in which state changes are handled and re-rendered. A React version of this progress bar element would simply have a render method, that when the state changed would simply be called again for another full-re-render of the element. The fact that this grabs individual elements and mutates them is about as far from React-style programming as you can get. Virtual DOM, tree-diffing, all of this is around a central concept that "immediate mode" programming reduces visual bugs, and is completely absent from custom elements. I'm not saying Custom Element are wrong, but they are absolutely different from React in a core and essential way.