Allow rem units in HTMLPurifier
Edit the $allowedUnits parameter in /Length.php
protected static $allowedUnits = array(
'em' => true, 'ex' => true, 'px' => true, 'in' => true,
'cm' => true, 'mm' => true, 'pt' => true, 'pc' => true,
'rem' => true // ADDED
);
