This may not be a complete answer but I think it's a useful pointer in the right direction.
There's a project called jqcaldav, which is (as the name suggests) not exactly an iCalendar (ics) viewer but a CalDAV client. So far I haven't found a way to specify an ics URL to display directly.
However, since the CalDAV protocol uses the iCalendar format to store and communicate Calendar data, the jqcaldav project should contain some code to parse and display iCalendar Data. Maybe examining the code can be a useful exercise and lead to the development of a standalone ics viewer. I would also be highly interested in a working solution, but so far I haven't had time to put something together myself.
A quick analysis shows that there are indeed two files that are seemingly concerned with the iCalendar format: ical.js and jical.js. Examining the source code reveals that another file called ics-parser.js is used. Also, the calendar solution was apparently released under a CC-By-License, encouraging reuse in other projects.
If someone creates a project based on this, I'd glad to know of it or even join in.
