[haskellshill]

Are you misunderstanding on purpose? I am aware they are optional. I am arguing that there is no reason to omit them from your HTML. Whitespace is (mostly) optional in C, does that mean it's a good idea to omit it from your programs? Of course a br tag needs no closing tag because there is no content inside it. How exactly is that an argument for omitting the closing p tag? The XML standard has no relevance to the current discussion because I'm not arguing for "starting to treat it like XML".

[recursive]

I'm beginning to think I'm misunderstanding, but it's not on purpose.

Including closing tags as a general rule might make readers think that they can rely on their presence. Also, in some cases they are prohibited. So you can't achieve a simple evenly applied rule anyway.

[haskellshill]

Well, just because something is allowed by the syntax does not mean it's a good idea, that's why pretty much every language has linters.

And I do think there's an evenly applied rule, namely: always explicitly close all non-void elements. There are only 14 void elements anyway, so it's not too much to expect readers to know them. In your own words "there's no substitute for actually knowing the real rules".

I mean, your approach requires memorizing for which 15 elements the closing tag can be omitted anyway (otherwise you'll mentally parse the document wrong (i.e. thinking a br tag needs to be closed is equally likely as thinking p tags can be nested)).

The risk that somebody might be expecting a closing tag for an hr element seems minuscule and is a small price to pay for conveniences such as (as I explained above) being able to find and replace a p tag or a li tag to a div tag.

[recursive]

I don't believe there are any contexts where <li> is valid that <div> would also be valid.

I'm not opposed to closing <li> tags as a general a general practice. But I don't think it provides as much benefit as you're implying. Valid HTML has a number of special rules like this. Like different content parsing rules for <textarea> and <script>. Like "foreign content".

If you try to write lint-passing HTML in the hopes that you could change <li> to <div> easily, you still have to contend with the fact that such a change cannot be valid, except possibly as a direct descendant of <template>.

[haskellshill]

Again, you're focusing on a pointless detail. Sure, I made a mistake in offhandedly using li as an example. Why do you choose to ignore the actually valid p example though? Seems like you're more interested in demonstrating your knowledge of HTML parsing (great job, proud of ya) than anything else. Either way, you've given zero examples of benefits of not doing things the sensible way that most people would expect.

[recursive]

To (hopefully) be clear, I don't think there are many benefits either way.