I have an online calendar in an RSS-format. Is is possible to import it into iCal and will it be kept synced automatically?
An example of such a calendar is: RSS-Calendar
Generally speaking Calendar supports CalDAV and Exchange calendars. I am not certain what an "rss-calendar" is but if it can be converted to or supports CalDAV or Exchange calendar formats you are golden.
I have not had to import a or subscribe to a calendar in quite a while so Calendar in the last couple of versions of Mac O/S may have some new supported formats. You may want to try just pointing Calendar to the URL of your "rss-calendar" and see what happens.
The other possibility is if you can export an ICAL compatible file that can then be imported into Calendar. It's not automatic syncing but it may be better than nothing.
As you can see from the example you provided, RSS feeds are not meant for calendars. Instead, they are meant for use with an RSS reader (like Safari) that will display, in your example, dates in text format and provide a "Read more..." link that will direct you to the website.
Unfortunately, there's not much you can do, because even converting the RSS feed for use with a calendar would mean you need to convert it every time the feed updates.