textarea user edit style

I think you looking for textarea (WYSIWYG) editors. You can find more by googling. Try these,

CKEditor

Demo: http://ckeditor.com/demo

TinyMCE

Demo: http://www.tinymce.com/tryit/full.php

For More, Check these links for more editors,

http://web.enavu.com/tutorials/14-jquery-and-non-jquery-rich-text-editors/

http://www.webdesignerdepot.com/2008/12/20-excellent-free-rich-text-editors/

reference:
https://stackoverflow.com/questions/15539166/html-text-area-with-custom-user-font-size-style-and-colour