There are two kinds of character codes: standard and HTML 4.01. Standard character codes are the most widely supported by various browsers. You can see a complete list of these at the HTML Appendix C page http://www.htmlref.com/Reference/AppC/standard.htm.

The HTML 4.01 character set expands on the standard set with additional character codes such as those for Greek characters and symbols, including those for mathematics, arrows, and others. It’s especially important to preview these codes in browsers to ensure that they display properly. You can see a complete list of HTML 4.01 codes and browser support at http://www.htmlref.com/Reference/AppC/html4char.htm.