The board.php file contains 24 <br>.
It seems the all the other files use <br /> (442).
Is phpBB suppose to be fully HTML5 or is it using XHTML on some pages?
If its only suppose to be HTML5 everywhere then perhaps only use <br>.
But <br> and <br /> is valid in HTML5 but personally i perfer the simplest.
XHTML uses <br />.
Additional <a href="">, <strong>, <em>, <u> and <br /> are allowed.
Perhaps that should be updated.