What's the point though to bake that into HTML if you need tons of JavaScript to make it happen anyway? You can just as well use the flexibility of JavaScript to build up component graphs. HTML, or rather SGML, has an established method of syntactically composing fragments using SGML entities that is well integrated into the process of editing HTML using plain text editors, not to speak of extending the vocabulary with new elements and their content models in powerful ways. Introducing elements into markup using script rather than declarative means only serves to complicate parsing, with questionable benefits.
Because with HTML you would use HTML to structure your elements. Javascript was never designed to structure element s we get things like JSX, to compensate, adding further complexity. HTML Imports was an attempt to keep with the existing platform.
