HTML keeps getting pulled in so many directions. I wish XHTML had won. It was modular, pluggable, extensible, and semantic. The last bit might have eventually made entering the search space easy for new competitors, too.
But the browser vendors failed to make a stand against bad HTML.
<img>
<srcset>
<source><width>1024</width><src>large-image.jpg</source>
<source><width>512</width><src>small-image.jpg</source>
</srcset>
<src>image.jpg</src> <*>fallback</*>
<alt>My image</alt>
</img>I dislike XML, the confusion between attributes and sub elements is one of the worst bits.
That is what your code would look like to browsers that didn't know about the new elements. HTML is defined such that browsers can ignore unknown elements for compatibility and still display the text. Using contents for the metadata means that browsers need to know about the elements to at least hide the text.
This is why *ML is a stupid-ass way to do things. "the problem it solves is not hard, and it does not solve the problem well."
JSON is great as an interchange format, but there are many reasons editing it by hand is painful, lack of comments and lack of newlines in strings not being the least of them.
HTML5 got one thing right though: standardization of the DOM failure behavior. As an implementation detail of their design, they went with "sensible recovery" for failures over stricter failure modes.
In going with the WHATWG over the W3C, we ultimately chose "easy to author, (slow to evolve) living standard" over "strictly typed yet developer extensible". I was disappointed, but it's good for some parties I suppose. (It certainly keeps the browser vendors in charge of the core tech...)
The W3C over-engineered to a fault. They had a lot of the right ideas, but were too enamored by XML and RDF.
No browser vendor was going to ship new features only in XML parsing mode, because that was author-hostile enough that it would lose them authors, and thus users. (Browser game theory.) The choice of HTML over XML syntax was purely practical, in this sense.
It was browsers that did that in the first place. HTML5 just standardized the exact behavior on failures.
I want to know more...
HTML 4 - vendors implemented the spec incongruently and failed in their own special ways. XHTML strict - standard parsing rules with strict failure mode. HTML 5 - standard parsing rules, suggested (but not required) rendering behavior for browser uniformity, and well-defined failure behavior.
There's something you're overlooking in the above. If a compiler was smart enough to know what to do with your erroneous code and compile in spite of the errors, that would be the end of programming and programmers.
I love it when my code doesn't compile (i.e. if I've made a mistake). Much worse if when something tries to be "intelligent" and makes my code do something I never asked for - then I spend hours trying to figure out what the issue is (assuming I've noticed) rather than seeing that I made a mistake and fixing it.
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.
If it were done right, anyone could have defined a markup module/schema with parsing rules and scripting. The evolution of those extensions would have been pretty damned fast due to forking, quick vetting/optimization, etc. It would have been well timed with the recent javascript renaissance, if it had happened. It might have meant browser vendor independence at the level of the developer.
HTML should really have been modular with an efficient, lightweight core spec. It should have also paid lots of attention to being semantic so that others could compete with Google on search. I am still curious if that's why Google got involved in the WHATWG. I'm rambling about things I don't know about though...
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.