You can replace the standard WordPress comment form with a built-in WYSIWYG editor TinyMCE.
Find the file where the comment form is defined. Usually it’s comment.php in your theme
For WP <3.0 it looks something like:
In WP3.0 a new function custom_form() (defined in wp-includes/comment-template.php) appeared.
Add the following before or after the form:
<script type=”text/javascript”
src=”<?php
echo (get_bloginfo(‘wpurl’));
?>/wp-includes/js/tinymce/tiny_mce.js”>
</script>
<script type=”text/javascript”>
tinyMCE.init({
theme : “advanced”,
mode : “textareas”,
plugins : “safari,autosave,media”,
languages: “en”,
theme_advanced_buttons1 : “bold,italic,underline,undo,redo,
link,unlink,image,forecolor,styleselect,
removeformat,cleanup,code,cite”,
theme_advanced_buttons2 : “”,
theme_advanced_buttons3 : “”,
theme_advanced_toolbar_location : “top”,
theme_advanced_toolbar_align : “left”,
theme_advanced_styles : “Code=codeStyle;Quote=quoteStyle”,
entity_encoding : “raw”,
remove_linebreaks : false,
inline_styles : false,
convert_fonts_to_spans : false
});
</script>
For other options, see the TinyMCE documentation and examples: http://tinymce.moxiecode.com/examples/example_01.php
Also, you will need to copy the original tiny_mce/themes/advanced/langs directory from the TinyMCE distribution into wp-includes/js/tinymce/themes/advanced. Otherwise you will not get the correct menu labels.