Something that looks like Present Perfect
I stumbled upon the following sentence: "I ett grynigt videoklipp som CNN tagit del av ses en ung pojke säljas till högstbjudande för gårdsarbete." I get that it means that CNN has taken part in a video, but I don't understand why there is no "har" between CNN and "tagit".
It is fine to omit "har" in subordinate clauses. This is perfectly grammatically correct and very common in formal texts. I can see that you take the courses in Norwegian and Danish as well so I might as well tell you that this "rule" does not apply in those languages - it's just us Swedes who are a bit lazy!
You're right that that looks like present perfect. Not sure but it could just be that you're meant to assume the "har." In practice, a lot of Swedish writing just... doesn't stick to the rules.
In my experience, Swedish has a lot of variance between written/spoken Swedish and the "standard" that is taught in classrooms (I'm not fluent though so it's very possible that I just don't know all the rules--I think the answer you gave on this discussion is a lot better than mine).
For example, it's common to see words like någon shortened to nån even in text, so it seems reasonable that words/letters/etc that could be assumed by a reader would be skipped over now and again. Since the secondary verb is conjugated in present perfect, you can assume the "har/hade" very easily.
That being said, I've never in my memory come across this particular construction before.
"Tagit del av" doesn't mean "taken part in". It's an expression that doesn't seem to have an exact counterpart in English.
On this page there are lots of examples of how "tagit del av" has been translated into English: