I am attempting to create a ICalendar with a html description using the X-ALT-DESC;FMTTYPE=text/html: but it does not appear to support inline CSS for layout/styling is there anyway of making CSS work within an icalendar?
RFC 5545 (the iCalendar specification) allows you to use HTML, but that's a different thing from the calendar clients supporting it. I suspect that you'll find it rare that any of the non-browser-based calendars will do so.