Outlook Web Access OWA can utilize HTML signatures but there are restrictions on the use of complex formatting. You cannot simply copy and paste a complex signature from Outlook into Outlook Web Access if the signature contains images, links and other complex formatting.
Doing so can break the web based options page.
If you get a pop up error from Internet Explorer on your OWA options page, it is probably caused by your custom signature. You will need to use Outlook Web Access Light (option at log in screen) to clear the bad signature.
Once in the light mode, you can choose in options to replace the current signature. Type something in the replace my current signature with... box and save the new signature. Then log out and back in under the normal mode. You can now use the built in editor to format your signature as you like. Type directly in the signature box and be very careful what you paste in there.
The above applies to Exchange 2007 Outlook Web Access, we're not yet certain if it's been corrected in Exchange 2010.