Somewhere between 30% and over half of all online ad "impressions" are registered when the ad is outside the browser window or otherwise unable to be seen by anyone, according to industry estimates. That's why the online ad industry has worked so hard over the past few years to establish a new metric for online advertising, the "viewable impression."

One problem: the standards organization Media Ratings Council (MRC) didn't accredit the metric and back in November 2012 told the advertising industry not to use it. That changed today when the MRC lifted its advisory, giving buyers and sellers of digital ads the all clear to do so for the first time.

The MRC's move brings the ad industry one step closer to buying and selling only those ads that make it "in view."

"By adopting this standard for viewable display impressions, the entire marketplace – agencies, marketers and publishers – will benefit from the improved quality and accountability of digital advertising," said George Ivie, executive director and CEO of the MRC. "This shift will ultimately benefit the entire advertising ecosystem by paving the way to better cross-platform campaign planning and analysis."

The lifted advisory applies to online display only. Video is expected to follow in the coming months.

Collaborating with the Interactive Advertising Bureau, the MRC released guidelines defining what "viewability" means, as well. For an ad to be considered "viewable," according to the guidelines, 50% of its pixels must show up in the viewable portion of a browser for at least 1 second.

The MRC did not previously believe the vendors measuring viewability were able to do so effectively. But that's changed.

"The state of viewability measurements is much better than it was 16 months ago and viewability vendors have greater insight into the range of ads that are served in campaigns," said David Gunzerath, SVP and associate director of the MRC, in an interview with AdExchanger.

The approved vendors on the MRC's list include: RealVu, comScore vCE-Validation, DoubleVerify, Google Active View, spider.io, Integral Ad Science, Alenty, Sizmek, Moat, WebSpectator and Glam Media.

The IAB seized on the moment, declaring it a "sea change"

Sherrill Mane, SVP research, analytics and measurement at the IAB wrote in a jubilant blog post that one of the major obstacles to bringing brand dollars online had been removed. "Publishers cannot compete for brand dollars on a level playing field if they cannot guarantee viewable impressions," she said.