> But programming has strict rules itself and will also fail if not adhered to so I never understood the complaint of "draconian" error checking in XML/XHTML.

In most cases, if your code has some syntax error, the author of the code sees the syntax error; in the web case, if your code has some syntax error, the user sees the syntax error. That's the dramatic difference.

The other reality is unlike program code, there's vastly more often user content intermixed in (X)HTML and it's rare for people to implement sanitisation correctly (do you handle U+0000? U+FFFF? U+1FFFF? most people outputting XHTML historically haven't, even if they get the security critical stuff (like "<") right).