When people ask questions and put HTML tags outside of code blocks one of two things happens.
It's a supported tag and it changes the formatting. This is usually OK, because it's easy to spot that there is something wrong with the question and someone edits it quickly.
It's not in the list and it gets silently ignored. This tends to lead to confusion and even high rep users can get confused by such questions and post irrelevant answers. Example (see edit history).
Couldn't unsupported tags then be escaped instead? What is the reasoning behind silently ignoring them?