<!-- Displays all of the WordPress default allowed HTML tags with attributes.-->
<p><strong>XHTML:</strong> You can use: <code><?php echo allowed_tags(); ?></code></p>

The allowed_tags() function is defined in wp-includes/general-template.php of your WordPress directory. This function calls the $allowedtags global variable defined in wp-includes/kses.php. The allowed_tags() function displays all of WordPress default allowed HTML tags with their respective attributes.

It is preferable to use CSS for styling as a better approach than using the <strong> tag.

In some themes, the allowed_tags() function is just commented out. In this case, all you have to do is Uncommenting that line of code, in order to show what HTML tags are allowed to use in the Comment textarea.

If the below line of code (or something similar) already exists in your theme´s comments.php file, then all you have to do is to Uncomment it (meaning remove <!–– from the beginning of the line AND ––> from the end of the Line. That´s ALL.

<!--<p><small><strong>XHTML:</strong> You can use these tags: 
<code><?php echo allowed_tags(); ?></code></small></p>-->

Note: The allowed_tags() function merely shows the user what are the allowed HTML tags for WordPress comment textarea. Commenting this function out or removing it from comments.php file does not prevent a knowledgeable WordPress user from using the default WordPress allowed HTML tags in the Comment textarea. This function merely displays the tag on the browser´s window, it does not prevent the user from using them. Hidden tags does not mean that they can´t be used.

In other themes and theme frameworks, you can display the allowed HTML tags in the comment section by changing the theme´s setting in WordPress dashboard. In this case, CODE-1 is NOT needed.
For example, for this blog I am also using the Clear Line Theme. The below image shows you where to change the setting.

In the case of this blog, which is beyond the scope of this tutorial, I did modify comments.php file for the Clear Line Theme, since I also wanted to show what shortcodes commenters can use.

For WordPress Version 3.3.1, the default allowed HTML tags list for comments are the following:
<a href="" title="" rel=""> <abbr title=""> <acronym title=""> <b> <blockquote cite=""> <cite> <code> <del datetime=""> <em> <i> <q cite=""> <strike> <strong>

The above list is what is allowed and displayed by default. The code that lists the above HTML tags in Comments is located in kses.php file in wp-includes folder of your WordPress directory. The code can also be found at WordPress Trac. The code is displayed below for your convenience (with some modifications from the original):

kses.php File :: CODE-default :: default $allowedtags global variable for the Comment Textarea :: WP 3.3.1

<?php
$allowedtags = array(
    'a' => array(
        'href' => array (),
        'title' => array ()),
    'abbr' => array(
        'title' => array ()),
    'acronym' => array(
        'title' => array ()),
    'b' => array(),
    'blockquote' => array(
        'cite' => array ()),
    'cite' => array (),
    'code' => array(),
    'del' => array(
        'datetime' => array ()),
    'em' => array (), 
    'i' => array (),
    'q' => array(
        'cite' => array ()),
    'strike' => array(),
    'strong' => array(),
	);
?>

How To: Add HTML Tags to the Allowed Default List

By default, WordPress prevents potentially harmful and/or invalid HTML tags and attributes from the Comments Section. But in some cases you might need to add some HTML tags to the allowed list. Meaning, you want to add HTML tags that are not originally defined in the $allowedtags list.

I would advice against adding any HTML tags to the allowed list not already approved by WordPress. There is a reason they are not allowed in the first place. But if you insist, open your theme´s functions.php file, and add (copy and paste) the following code.

CODE-2A :: functions.php File

<?php 
/************************************CODE-2A*******************************
* @Author: Boutros AbiChedid 
* @Date:   March 20, 2012
* @Websites: bacsoftwareconsulting.com/ ; blueoliveonline.com/
* @Description: Allow <a href="" title=""> <blockquote cite=""> <cite> 
* <code> <em> <strong> <pre> <p> tags/attributes in the Comments textarea section.
* @Tested on: WordPress version 3.3.1 
* FEEL Free to MODIFY The Code to fit your needs.
* You can add other HTML tags (and their attributes) this way. But don't 
* push it, there is a reason they are not allowed in the first place.
* @DO NOT REMOVE THIS SECTION (See Blog's terms of use.)
**************************************************************************/

function bac_allowed_html_tags_in_comments() {
  define('CUSTOM_TAGS', true);
  global $allowedtags;
 
  $allowedtags = array(
      'a' => array(
           'href' => array (),
           'title' => array ()),
	  'blockquote' => array(
		   'cite' => array ()),
	  'cite' => array (),
	  'code' => array(),
	  'em' => array(),
	  'strong' => array(),
      //Notice that <p> and <pre> were not allowed by default in kses.php file.
      'pre' => array(),
	  'p' => array()
  );
}
//Hooks the function to the specific action (init).
add_action('init', 'bac_allowed_html_tags_in_comments', 10);
?> 

CODE-2A Notes:

The <pre> and <p> tags are just an example. These tags were not included in the default list (in CODE-default). You can add any tags you like, but as I said, I would not advice this, unless you really have to.

You can use either CODE-2 or CODE-2A but NOT both.