[dfabulich]

I think the industry settled on pretty good answers, using lots of XML-like syntax (HTML, JSX) but rarely using XML™.

1. Following Postel's law, don't reject "invalid" third-party input; instead, standardize how to interpret weird syntax. This is what we did with HTML.

2. Use declarative schema definitions sparingly, only for first-party testing and as reference documentation, never to automatically reject third-party input.

3. Use XML-like syntax (like JSX) in a Turing-complete language for defining nested UI components.

Think of UI components as if they're functions, accepting a number of named, optional arguments/parameters (attributes!) and an array of child components with their own nested children. (In many UI frameworks, components literally are functions with opaque return types, exactly like this.)

Closing tags like `</article>` make sense when you're going to nest components 10+ layers deep, and when the closing tag will appear hundreds of lines of code later.

Most code shouldn't look like that, but UI code almost always does, which is why JSX is popular.