That’s entirely down to < and > being the delimiters for HTML commands.
Some of which Discourse respects and passes along to the browser verbatim, like superscript and subscript, whereas others are suppressed like. And others are simply passed through to the browser which ignores them as nonsense, leaving nothing visible in that spot. The word “stuff” here is inside <abcd> tags which are meaningless in html and so although Discourse faithfully sent them to your browser, it ignored them as nonsense and displays nothing.