Allowing more elements in comments via functions.php

Here is an example how to allow a commenter to insert HTML5 video into the comment. Both <video> and <source> elements has two allowed attributes. preprocess_comment filter is applied when saving the comment to the DB.

See /wp-includes/kses.php for $allowedtags array structure.

function myAllowHtmlComments($comment) {
    global $allowedtags; 
    $allowedtags['video'] = array(
        'width' => true,
        'height' => true
    );
    $allowedtags['source'] = array(
        'src' => true,
        'type' => true
    );
    return $comment;
}
add_filter('preprocess_comment','myAllowHtmlComments');