By default the WordPress comment box does allow basic html tags, but most bloggers don’t’ know that and most commenters don’t know html. So what is a quicktag? Well this is a quicktag its a button that users can use to easily and quickly style their comments.
To get that on your site, install and activate the basic comment quicktags plugin.
Then to go Settings — > Discussion.
And click activate quicktags for comments.
Save your changes.
Now your commenters will have these quicktags to use while responding to your posts.