GroupM and Conde Nast caught a lot of people off-guard when the publisher agreed to only charge the media agency's clients for ads that were guaranteed to are entirely viewable by comsumers, a tougher stance than the industry was pushing.

"I got emails from all of my peers in the industry saying, 'What did you do? Are you crazy?'" Conde Nast's senior VP-digital sales Lisa Valentino said on Tuesday at the first day of Ad Age's Digital Conference in New York.

However the agreement may not be as crazy as it sounds. For starters, it only applies to industry-standard banner ads, the typical squares and rectangles adjoining articles on publishers' web pages, according to GroupM's chief digital officer, Rob Norman, who joined Ms. Valentino and Ad Age's Michael Sebastian on stage. "This is only dogmatic within that narrow prism of IAB standard units," he said.

That means GroupM's clients usually still have to pay for so-called "native" ads, sponsorship deals and ad formats custom to Conde Nast, even if those non-standard ads don't meet the industry's viewability standard.

"The deal is not intended to create issues for those types of really high-end, high-quality executions," Ms. Valentino said.

"If one of [GroupM's] clients is doing a huge sponsorship on one of our sites, it may not be 100% in view," she added. "That's not the intention of this relationship."

Mr. Norman said GroupM and Conde Nast reached an "acceptable [viewability] level for those kinds of units."

Mr. Norman also addressed skepticism that 100% viewable campaigns aren't realistic because they may cut publishers' inventory levels to the point of shrinking their revenue. "No chance of that happening," he said. He added, "The publisher community and agency community will coalesce around a very high bar for viewability. ... It will happen, and it will happen very quickly."

And the reason it will happen? "The advertiser has a history of understanding they get what they pay for. When anything happens [that jeopardizes that], the market is corrupted," Mr. Norman said.