He is perfectly following his own advice. Apparently, his form field takes HTML syntax with a subset of HTML tags. Your input does not conform to that. So, instead of silently altering what you wrote, it tells you about the validation failure and asks you to correct your input instead of silently changing what you wrote. The input field takes HTML, so you have to write "<script>" (I suppose, haven't tested it) in order to display "<script>" - that is perfectly consistent.
No form should accept just "HTML" if you don't want just any "HTML" in your form.
I was actively trying to talk about his script example and instead I had to second-guess his parser to get past the validator (I eventually resigned and replaced < and > with [ and ]).
If you want to support some tags, have your parser be an HTML-like DSL language with those tags supported. Don't disallow perfectly good input.
I don't really understand what point you are trying to make, but in any case, he has a particular input format for that form field, your input did not conform to it, so it was rejected, nothing particularly surprising or wrong there.
Now, I haven't tried it, but I suppose his form field expects HTML syntax? Have you tried entering your text in HTML syntax? Was that rejected?
