[AgentME]

You already have to consider all of those cases about the <p> tag: because they auto close when they hit one of those elements, that means that <p> tags can't contain any of them. If you don't know about this while using <p> tags, you can be in for a world of fun mysterious issues.

[luhn]

But all those tags are things that no sane developer would put inside a p tag anyways, so you don't really have to think about them.

The real mental overhead is incurred when reasoning about the tag following the p, which could be anything. "Hmmm, I have a nav tag coming after this p tag. Does that implicitly close it?"

Although if you had a good autoindenter, you could catch any mistakes by how it was indented. "Oh, that nav tag is on the same indentation level as the p tag, I guess it does implicitly close it."

[tinco]

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.

[sopooneo]

Because the world is made up of messy people. And the value of allowing messy content was perceived as outweighing the value of consistency and reliability. I happen to agree.

[AgentME]

I ran into this when working on some software that put user comments in <p> tags. I added some allowed markup that came out as <div> tags for a collapsible section. It didn't strike me as a particularly insane feature, but I about lost my mind trying to figure out why the <div> tags appeared to negate the <p> tag styling for all of the text after it.