I may have been stuck in xml model or several years ago but in the Uni course I learned that a HTML5 document doesn't validate if you close self-closing elements. You should write <img src="" alt="">, <br>, <hr> etc. and not <br/> and so on.
Here's the W3C HTML5 validator: https://validator.w3.org/nu/#textarea. You can also add an extension to your code editor.
Also, the closing tag may be omitted if a block element follows, for instance:
<p>Some text added
<ul>
<li>item 1</li>
<li>item 2</li>
</ul>
is valid.
Confusing, eh!