Wheyll....
When you go to edit a signature, there is a button/panel that lets you switch to formatted (rich) text and apply formatting: bold, italic, underline, font color, font size.
Formatted (rich) text is then sent using HTML.
Not sure I understand your comment about HTML tags.
By that logic, Gmail (web), or Yahoo, or Hotmail don't support formatted text either.
They have buttons for bold, italic, etc, but how do I enter something like:
<div style='padding: 3em; font-weight: 700; font-family: Comic Sans'>
Don't see a way to do that.