This has been bugging me for quite some time now and since I started using HTML a long time ago in a galaxy far far away <br> was the tag format of choice. Actually it was the only choice… Then I started seeing <br>, <br/>, and <br /> sprinkled everywhere. It is amazing how having to type one extra keystroke for that one slash can be so annoying. So it’s good to know that according to:
Void elements (such as br tags) only have a start tag; end tags must not be specified for void elements.
Optionally, a “/” character, which may be present only if the element is a void element. So you can use <br> or <br/> or <br />
So unless you are trying to satisfy other languages such as XHTML or XML generating HTML, you can use any one of the above and it should still validate (https://www.w3.org/)