By SYDNEY EMBER

New York Times

Ads pop up and play automatically, daring readers to shut them down with feats of fine motor control. The ads commandeer the screen. They expand and contract. They cover the text and refuse to budge.

And then there is the dreaded X — the one that invites you to close the ad yet seems impervious to repeated clicks of the cursor or the jabs and thrusts of even the most powerful fingers. (Perhaps you have tried a hammer?)

Sometimes the ads dance and move across the screen, forcing the user into a hot pursuit of the X.

“How many times have you hit the X and it doesn’t work?” said Tony Weisman, chief executive of the digital agency DigitasLBi North America. “Now it’s just a cruel joke.”

Online advertisers and consumers have tried to outmaneuver each other since the early days of the Web — with sellers continually finding ways to prolong engagement with ads and users trying equally hard to avoid them. But the cat-and-mouse game has reached a critical point, especially as devices have gotten smaller: Ads have become so annoying, consumers and industry executives say, that they could sink the Internet if they were not also helping support it.

“Ads are getting more pervasive and more difficult to easily get past,” Weisman said. “We are just destroying the user experience.”

On desktop computers, ads have turned Web pages into mazes. A recent visit to Salon.com, for example, meant fending off expanding banner ads and navigating video ads that played automatically. Sponsored posts and animated display ads filled each page.

On tablets and smartphones, the problem is more acute. Ads dominate the smaller screens, and many ads are not formatted correctly because of out-of-date technical language. The X button can be so tiny that clicking it requires a fair amount of luck. Industry executives often cite a 2012 report that said up to 50 percent of advertising clicks on mobile were accidental.

Annoying ads have become problematic for Anthony Martin, a 32-year-old consultant for a project management firm who sat in Bryant Park on a recent Monday afternoon, iPhone 6 in hand. He had moved to New York not long ago, he said, and was using a smartphone app to determine the best subway routes. But as soon as the app loads, ads take over his screen — first a banner ad on the bottom, then a full-screen ad. No amount of desperate jabbing does the trick.

“Sometimes I miss a stop,” he said. “Especially with fat fingers.”

Industry executives say it is quite likely that publishers and mobile developers are deliberately building ads that are hard to escape or shut down.

“The ones that are incredibly invasive are designed to be that way,” said Brian Gleason, global chief executive of Xaxis, a media and technology company owned by advertising giant WPP.

Mike Pilawski, vice president for product at Vungle, which builds and serves mobile video ads, says some advertisers ask Vungle to make the whole screen clickable at the end of the ad — not just the X or other specific buttons — which would make the ad difficult to close. He says Vungle refuses to do that, but it does design ads with X buttons in the top left instead of the usual top right. The switch confounds some users, though he insists that is not the intent.

Bad Web ads have deep roots. According to Ethan Zuckerman, who helped invent the pop-up ad in the 1990s, those behind the early Internet turned to advertising out of necessity.

“There were so few people online, and the Internet was so new, and everybody still thought it was a fad,” he said. “It would have been hard to convince anybody to put money down.”

Pop-up ads were intended to give advertisers a way to separate their message from the Web page. (This became especially important, Zuckerman’s story goes, when a car company became upset after it bought a banner ad on a page about anal sex.)

Advertisers soon began tracking Web users and serving them targeted ads. Later, companies like Rocket Fuel and Rubicon Project started selling technologies that automated this process, which eventually helped decrease the price of online ads.

But cheap online ads squeezed publishers, who responded by cramming more aggressive ad formats onto their sites to secure enough revenue to stay alive.

Online readers end up caught in the middle, forced to withstand a constant stream of intrusive ads. Making matters worse, the ads and the technology behind them are bogging down the Web. The worst ads load so slowly that they use up data plans and sap battery life.

Ghostery, which makes a plug-in that identifies and blocks online tracking tools, reported 76 trackers on Boston.com’s home page one day last month, with names like Datalogix, LiveRail and NetSeer. (The New York Times had 39 on the same day.)

“The publishers have been forced into situations where they will allow much more than they used to,” said Gleason, of Xaxis.

On The Atlantic’s site last month, for example, a full-page video ad for the aircraft manufacturer Airbus played automatically, looped repeatedly and required readers to “Scroll down to continue to The Atlantic.” Some ad executives wondered if the ad — aimed, apparently, at the elite few who are in the market for planes — was placed in error.

A spokeswoman for The Atlantic, however, dismissed the possibility, writing in an email that the ad was sold directly and was intended “to reach The Atlantic’s large and highly influential audience.”

Many Web users are willing to put up with the ads to get the content they want or to find and buy the merchandise they are seeking. Ads do serve a purpose beyond just stoking frustration.

But many consumers are fighting back. A rising number are installing ad blockers on both desktop and mobile. In September, Apple enabled ad blocking apps in its new mobile operating system, iOS 9, and many soared to the top of its App Store chart. Google blocks Flash ads on its Chrome browser.

Most industry executives recognize that bombarding consumers with intrusive ads is potentially undermining the business model that supports much of the Web. Larry Page, chief executive of Alphabet, Google’s parent company, recently said the industry needed to produce ads that were “less annoying” and “quicker to load” — and many publishers are reconsidering their advertising models.

Eleanor Cleverly, general manager of Boston.com, said the site was reducing the number of display ads on its pages and adding more native ads and sponsored articles. It has started stripping away ads on its mobile site. But for Boston.com — and many other publishers — simply doing away with ads is not a viable solution.

“If we’re going to offer our editorial news reporting for free, we’re going to have some sort of sponsor for that,” Cleverly said.

Still, she added, “I don’t think begging a reader to keep ads on the page is any conversation we can start.”

© 2015 The New York Times Company