[rezmason]

I'm exaggerating; Adobe Flex's MXML was an XML schema for building application UIs declaratively. It supported custom elements, which would correspond to classes you'd write, whose constructors, fields and methods would mirror the attributes and events on the DOM, kind of like a stricter HTML5.

That's a good description of the Flash ecosystem from 2006 onwards, actually: before its time, and more stringent than what came after. That strictness was the legacy of ECMAScript 4, I suppose.