You can not use HTML in the body of the message, but you can use it in
HTML file attachments when using an email program/service.
These HTML tags worked: <a>, <b>, <center>, <h1>, <hr>, <i>, <p>.
These HTML tags were removed: <bgsound>, <embed>, <iframe>, <object>,
<script>.
Because you can't use for example <embed> and <script> tags, you can't
include embedded videos or JavaScript code to your messages. But you
can use "inline style CSS" (CSS that is defined for example in a
paragraph tag).
List of tests:
HTML, CSS, and JavaScript Test (Message Body)
http://groups.google.com/group/Groups-Test-Group/msg/b32ef79004fcd125
HTML, CSS, and JavaScript Test (File Attachment) (Gmail)
http://groups.google.com/group/Groups-Test-Group/msg/1d00769c5aad4f3e
HTML Tags Test
http://groups.google.com/group/Groups-Test-Group/msg/c9e5c3724166e706