WordPress, by default only allows <a> tag with href and title attribute; <abbr> and <acronym> tags with title attribute; <blockquote> and <q> tags with cite attribute; <b>, <cite>, <code>, <del>, <em>, <i>, <strike> and <strong> tags to be used in comment.
The example below will overwrite the default and only allows <a> tag with href and title attribute; <strong>, <em>, <code>, <ul>, <ol> and <li> tags to be used in comment (paragraph and line breaks automatically).
Add the following lines to your WordPress Theme’s template functions.php file (For example, functions.php file for a Theme named “default” would probably reside in the directory wp-content/themes/default/functions.php) or create it as a Plugin by filling the additional Standard Plugin Information:
<?php
$allowedtags = array(
'a' => array ('href' => array (), 'title' => array ()),
'strong' => array(),
'em' => array(),
'code' => array(),
'ul' => array(),
'ol' => array(),
'li' => array()
);
?>
To display all of the current allowed tags for comment within the comment area, edit your WordPress Theme’s template comments.php and add allowed_tags() template tag.
For example:
<p>You can use these tags: <code><?php echo allowed_tags(); ?></code></p>