The WHATWG spec includes a special rule for handling </br>, in the section on parsing when "in body":

An end tag whose tag name is "br"
Parse error. Drop the attributes from the token, and act as described in the next entry; i.e. act as if this was a "br" start tag token with no attributes, rather than the end tag token that it actually is.

The result is that invalid HTML like Hello <br>World</br>! will be rendered by browsers as though it had two linebreaks, Hello <br>World<br>!. This library currently (quite reasonably!) removes the erroneous end tag instead, giving Hello <br>World!

I'm using this library for processing some messy HTML, and it would be useful to have this rule match the spec / browser behaviour.