Fun fact: HTML void elements came first.

XML brought in "empty elements" by adopting SGML's "null end-tag" (NET), part of the SHORTTAG minimization (though it changes the syntax slightly to make it work).

In SGML's reference concrete syntax, the Null End-Tag delimiter is `/`, and lets one omit the end tag.

    <p>This is <em/very interesting/.</p>

Here the EM surrounds "very interesting." In XML the "Null End-Tag Start" becomes "/" and the "Null End-Tag End" is ">".

So in XML a `<br />` is syntax sugar over `<br></br>` and identical to it (an element with _no_ child nodes), but in HTML `<br>` is an element which _cannot contain child nodes_ and the `/` was never part of it, required, or better.

As called out in another comment, the trailing `/` on HTML void elements is dangerous because it can lead to attribute value corruption when following an unquoted attribute value. This can happen not only when the HTML is written, but when processed by naive tools which slice and concatenate HTML. It's not _invalid_ HTML, but it's practically a benign syntax error.