XML has the concept of self-closing tags, where a tag can end with a / and then omit its closing tag, containing no content beyond the tag name and attributes. While it share a lot of DNA with XML, HTML doesn't actually support self closing tags, ignoring any slash at the end of a tag. These pairs are all semantically identical.
<br>
<br />
<img>
<img />
<div>
<div />
That the first two are HTML void tags, meaning that they do not need a closing tag, so self-closing the tag is just a style choice. The last one is a problem, since div is not a void tag. This is the problem that was discovered by Rich Harris in his issue Svelte parses HTML all wrong.
I cannot imagine a sane human who would intend for <div /> to mean "the start of a div", and I don't not think we should enable this. Ideally I would want warnings or errors to appear in all cases where this would matter.
An interesting detail is that the general way self-closing tags are used in HTML is slightly different from in XML. XML traditionally does not put a space before the closing tag.
<xmltagname/>
<htmltagname />
In XML, it would generally be formatted like
<this/>, without the space before the/, but Netscape Navigator 4 couldn't cope with<input type="text"/>, where the/immediately followed an attribute, so the spec recommended a space before the/.
The case against self-closing tags in HTML
This style difference is still present to this day, even in HTML derivations that do not require it like JSX.