I didn't say it's a syntax error. I said it is not specified in the standard. You can put the slash there but it has no meaning, it does nothing, and browsers ignore it. If anything, it can cause problems: https://github.com/validator/validator/wiki/Markup-%C2%BB-Vo...

So it's best to not do it altogether. If one thinks they need to be compliant with both HTML and XHTML, I'd bet they have far more problems with the rest of their markup that would fail at such an effort.