> There's a reason almost nobody serves XHTML with the application/xhtml+xml MIME type, and that reason is that getting a “parser error” (this is what browsers still do! try it!) is always worse than getting a page that 99% works.

That’s not the reason almost nobody serves XHTML.

The real reason is Internet Explorer. Okay, it’s a little more nuanced than that, but I think it’s accurate enough. Microsoft killed XHTML by inaction.

It’s 2004. XHTML is now a few years old, and all the rage. You decide to use it for your new project which you’re developing. At the start, you serve pages as application/xhtml+xml, and that works well in Firefox; but you know that won’t work because Internet Explorer still doesn’t support XHTML, and 90% of your viewers will be using that. So, a little frustrated, you serve your nice XHTML as text/html. You still validate it manually for a while, but then that habit disappears. Eventually you make one or two small mistakes that would have been caught easily if it were parsed as XML—but it’s not, because of Internet Explorer. Over time this disparity grows.

People have been complaining of the inefficacy of XHTML for this exact reason for two or three years by this point.

It’s 2006. XHTML is acknowledged to have failed. Everything else supports it, but as long as IE doesn’t, you can’t serve as application/xhtml+xml, and so you can’t get the advantages of XML syntax.

Seriously, early failure is good—so long as you’re working with it from the start. The problems only occur when you try to add strictness later.

Just look at typing in code bases. Adding strictness to existing JavaScript or Python or Ruby? Nightmare. Starting with static types? Somewhere between fine and extremely desirable.

(I might be overselling strictness’s popularity at the time—people don’t always like what’s good for them. We’ve largely realised now that unfettered dynamic typing is a bad idea, but ten years ago that was not settled. People get used to things. If IE had permitted XHTML early on, people would have got used to the idea of XHTML’s strictness and, I think, got to mostly like it.)

XHTML did not fail because of XML’s catastrophic parse failure mode. It failed because HTML already worked, and Internet Explorer took way too losng to accept XHTML. If you’re forking the web and compatibility with existing documents is not a goal, you can’t use XHTML’s failure as an argument: it failed because of compatibility issues.

Well, Internet Explorer did eventually support application/xhtml+xml: in 2011, IE9. Way too late to matter. And so only by around 2015 or 2016 could you finally serve with XML syntax. And now why would you? For your system is big and has tiny errors here and there and your CMS just drops markup in and never got round to validating it and and and and so on. By that time, HTML had given up on the XML path, and although it worked, the momentum was entirely gone, so you’d run into difficulties due to inadequate documentation, inferior tooling (ironic), and various more.