Sidewinder supports the addition of processors that handle non-core mark-up languages, such as XForms, SVG and MathML.
XForms functionality can be added with the formsPlayer XForms processor from x-port.net.
If you have Sidewinder and formsPlayer installed, you can check that they are working by pointing Internet Explorer at the XForms samples on the Mozilla site, which are delivered as application/xhtml+xml. Sidewinder will be opened in embedded mode, with the XForms rendered correctly.
(The calculator, Flickr and tax forms work pretty much as in Mozilla, but there is a problem with how Sidewinder interprets the insurance form.)
SVG functionality can be added with the Adobe SVG Viewer.
MathML functionality can be added with MathPlayer from Design Science.