Comment 1 for bug 717538

This is by design for now. Originally is supported both HTML and Webkit however, items in HTML notifications far succeed that of what libnotify can handle (Multiple images, tabs, colorization, HTML Anchor links) so the decision was made to leave HTML Stylized notifications alone. If libnotify or another notification solution comes along that supports more extensive markup we can re-evaluate.

Also, not far out of the realm of possibility, but future releases of this plugin will have a settings page where this option could be enabled in (Parse/don't parse HTML notifications)