I have done web dev on and off for over 15 years and I've never even thought about what happens when you put a h1 in a p. In my opinion the browser should crash

I have done web dev on and off for over 15 years and I've never even thought about what happens when you put a h1 in a p. In my opinion the browser should crash and the operating system should BSOD. I have always been severely annoyed by the amount of shit browsers put up with. I don't understand why XHTML strict didn't get the traction it deserved and why they didn't continue along that line with HTML5.