As online video advertising takes off, pre-roll spots have become a source of ad revenue for websites - but mini rebellions may result in losing site visitors to rivals.

For that very reason, some video sites, including YouTube and Google's, don't run pre-roll ads - though others, including Yahoo, AOL, MSN and MTV's Overdrive, are happy to take in pre-roll ad revenues, writes the Wall Street Journal. Google, for one, doesn't sell any video ads on Google Video, though it has tested post-roll spots.

Pre-rolls are attractive to advertisers - and annoying to viewers - because they can't be fast-forwarded through. That's why sites use various tactics to keep annoyance at bay. AOL, for example, says spots not longer than 15 seconds work better and users shouldn't be subjected to more than one in a single video-watching session.

But online video advertising is still evolving - and though pre-rolls may not go extinct, they won't constitute the dominant species of online video ad, according to Yahoo's chief sales officer, Wenda Harris Millard.