> Responsive images could have been an XHTML module with a javascript implementation. The browser vendors could catch up and provide native implementations in their own time, but that would not postpone immediate usage.
This is exactly what happened, except without the XHTML nonsense. JavaScript polyfills of the picture element were created and in use before native implementations eventually caught up. (And native implementations are very necessary, in this case, because they need to hook in to the preload scanner, which is not JS-exposed.)
More generally, custom elements and extensible web principles in general enable all of this. Again, without XML being involved.
This is exactly what happened, except without the XHTML nonsense. JavaScript polyfills of the picture element were created and in use before native implementations eventually caught up. (And native implementations are very necessary, in this case, because they need to hook in to the preload scanner, which is not JS-exposed.)
More generally, custom elements and extensible web principles in general enable all of this. Again, without XML being involved.