Dude I lived in that world. A fair amount of developers explicitly opted into strict parsing rules by choosing to serve XHTML. And yet, those developers who opted into strict parsing messed up their XML generation frequently enough that I, as an end user, was presented with that "XML Parse Error" page on occasion. I don't understand why you'd think all developers would stop messing up if only strict parsing was hoisted upon everyone rather than only those who explicitly opt in.
> In your hypothetical world, they are making that syntax error… and just deploying it anyway.
No, they're not. In my (non-hypothetical, actually experienced in real life) world of somewhat wide-spread XHTML, I'm assuming that developers would make sites which appeared to work with their test content, but would produce invalid XML in certain situations with some combination of dynamic content or other conditions. Forgetting to escape user content is the obvious case, but there are many ways to screw up HTML/XHTML generation in ways which appear to work during testing.
> We have strict syntax almost everywhere. How often do you see a Python syntax error in the backend code?
Never, but people don't dynamically generate their Python back-end code based on user content.
> How often do you run across an SVG that fails to load because of a syntax error?
Never, but people don't typically dynamically generate their SVGs based on user content. Almost all SVGs out there are served as static assets.
