You don't have to write the XML tags while you type. The hardest part about writing long-form technical prose is writing long-form technical prose. You can concentrate on getting that right, in the editor of your choosing, using placeholders for diagrams and layout elements. Then, in a second pass, mark the text up.

Indeed. And it's odd no one's mentioned Pandoc yet.

Millions of websites are made with writing HTML by hand and it's certainly not that bad.

What would you use instead that would allow you to define your own markup? LaTeX? \em{Hello} isn't much better than <em>Hello</em>.

Sure, a single small example looks almost (but not quite) as bad, but in larger texts LaTeX is basically the text-you're-writing, plus some control stuff, whereas XML interferes with everything.

On non-English keyboards it is even worse.