It's YouTube. Whether you get no ad, an ad skippable after 5 seconds, an un-skippable 15 second ad, or an un-skippable longer ad varies. I'm currently getting a skippable after 5 seconds ad.
This may also depend on what ad blocker you are using. At least one popular ad blocker [1] sometimes fails to block the "skippable after 5 second" ads but DOES block the "skip this ad" overlay. To make that even more annoying, I think that the "skippable after 5 second" ads are often long ads so the result is people without an ad blocker get 5 seconds of the ad, and people with that ad blocker get get the full thing!
[1] I'm not naming it because I was trying out different ad blockers on different machines to see which met my needs best, and I don't recall for sure which one it was.
This may also depend on what ad blocker you are using. At least one popular ad blocker [1] sometimes fails to block the "skippable after 5 second" ads but DOES block the "skip this ad" overlay. To make that even more annoying, I think that the "skippable after 5 second" ads are often long ads so the result is people without an ad blocker get 5 seconds of the ad, and people with that ad blocker get get the full thing!
[1] I'm not naming it because I was trying out different ad blockers on different machines to see which met my needs best, and I don't recall for sure which one it was.