Open-web display is advertising delivered in image or text units on websites that depend on third-party technology to sell their ad space. The definition is not only descriptive. It is the boundary of a legal market. Judge Leonie Brinkema, in the memorandum opinion issued on April 17, 2025 in United States v. Google LLC, described open-web display ads as display ads running on websites that use third-party ad tech infrastructure to match advertisers' ads to publishers' inventory. Almost every site carrying advertising falls inside that boundary, because building an auction is not what newspapers, recipe sites or trade publications exist to do.

The counterpart is the walled garden, which the same opinion defines as a publisher controlling the infrastructure through which advertisers buy and place advertisements on its own properties. Amazon, Google, Meta and Microsoft are named as the firms with the commercial scale and technical depth to sustain that arrangement. An advertiser reaching Instagram users has one route, which is Meta's own toolset. An advertiser reaching a regional newspaper's readers passes through a demand-side platform, an exchange and an ad server that neither the buyer nor the seller owns.

The path of an impression

The unit of trade is the impression: one opportunity to place one advertisement in one slot in front of one person. When a browser requests a page, the publisher's ad server sends a bid request to one or more exchanges. Each exchange runs its own auction, soliciting bids from demand-side platforms such as Display and Video 360 or The Trade Desk and from the advertiser side of ad networks such as Google Ads. The exchange returns its highest bid to the ad server, which screens out anything below the publisher's floor price and selects a winner. Trial testimony from Magnite put the window at 250 milliseconds per auction, repeated billions of times an hour.

Since 2015 the sequence has usually been preceded by a wrapper. Header bidding places a script in the page that solicits real-time bids from several exchanges before the ad server is called, and passes the winner in as a floor. Prebid, the open-source project that grew out of the technique, remains the standard implementation and is governed by publisher and vendor committees rather than by any single platform.

Fees are levied at each hop and are structured differently by layer. Publisher ad servers charge a flat fee per impression sold, working out at roughly 1% to 2% of revenue for large publishers. Exchanges charge a percentage of the winning bid, typically 10% to 20%. Trial testimony placed the combined cost of the chain at between $20 and $40 of every $100 an advertiser spends. Individual impressions sell for well under a cent, which is why the trade quotes everything as a cost per thousand.

The creative is deliberately dull. Standard units follow IAB sizes such as 728 by 90 and 300 by 250, and the category also absorbs outstream video, meaning video clips that play inside a display slot rather than inside a video player. That format distinction matters commercially: outstream has earned CPMs several times those of standard banners, which has produced a persistent incentive to misdeclare placement type.

From reservation to remedy

Early web advertising was sold by negotiation. Publishers booked reservations with agency buyers at fixed prices, a model that left large volumes of inventory unsold and gave rise to ad networks that aggregated the surplus and resold it.

Google entered in 2000 with AdWords and added the publisher side in 2003 with AdSense, by which point the advertiser base had passed 100,000. The structural moment came in 2008, when Google bought DoubleClick for $3.1 billion. DoubleClick's ad server, DFP, already held about 60% of publisher ad serving and counted nine of the top ten United States websites as customers. The deal also brought a nascent exchange, AdX.

Two policies then bound the pieces together. AdWords demand was permitted to bid almost exclusively into AdX, and publishers wanting real-time AdX bids had to run DFP. A feature called First Look gave AdX a right of first refusal on every DFP impression. By early 2014 AdX was winning 53% of the inventory DFP put to auction.

Publishers responded with header bidding, which achieved broad adoption among large sites during 2015 and, according to internal Google correspondence cited at trial, sharply increased their revenue. Google answered with Open Bidding in 2018, with Last Look, which let AdX see and beat the header bidding winner, and with Project Poirot, which shaded Display and Video 360 bids on rival exchanges. Last Look was withdrawn in 2019 alongside the introduction of Unified Pricing Rules, which barred publishers from setting higher floors for AdX than for other exchanges, and the same year Google moved Ad Manager to a unified first price auction.

Brinkema found in 2025 that this sequence amounted to willful monopolisation of the publisher ad server and ad exchange markets for open-web display, plus unlawful tying. The plaintiffs' third claim, covering advertiser ad networks, failed. The European Commission reached a parallel conclusion on September 5, 2025, fining Google 2.95 billion euros in Case AT.40670 and publishing a redacted decision on January 14, 2026.

A shrinking share of a growing market

The category matters because it funds the part of the web that is neither a platform nor a paywall, and because it is contracting while digital advertising overall expands. IAB and PwC put United States internet advertising revenue at $294.6 billion for 2025, up 13.9%, with programmatic at $162.4 billion.

Google's own filings tell the opposite story for the open web. Arguing against divestiture, the company told the court that open-web display was already in rapid decline, citing internal data showing that open-web display accounted for 11% of display impressions bought by AdWords advertisers in January 2025, against more than 40% in January 2019. Alphabet's Network segment, the line that pays third-party publishers, fell 4% to $6.97 billion in the first quarter of 2026 and declined again in the second.

Supply is falling faster than price. Ozone cohort data showed programmatic spend down 30.6% year on year across the first half of 2026, with a 44% drop in the United States against 14.3% in the United Kingdom, while average eCPMs rose about 30% in the United Kingdom and 7% in the United States, a pattern publishers read as scarcity rather than recovery. Within the format, demand is separating: AdRoll recorded retargeting CPMs up 18% and prospecting CPMs down 11% in January and February 2026.

Where the definition is contested

The market boundary is the most disputed part of the term. The Information Technology and Innovation Foundation argued that segmenting open-web display excludes mobile app inventory, walled gardens and non-display formats, noting that 55% of display spend in 2022 was for mobile apps. Google made the same argument at trial and lost on it, though it prevailed on the advertiser-side claim.

Quality is the second dispute. Made-for-advertising sites, built to generate impressions rather than readership, are estimated by the Association of National Advertisers to absorb roughly 15% of programmatic spend, and Integral Ad Science has put annual waste at about $10 billion. The exposure is uneven: mobile web display carried a 2.0% MFA rate against 0.5% on desktop and generated 71.9% of all measured MFA impressions.

Then there is the buyer's own reluctance. Survey work commissioned by Taboola found that 81% of respondents would raise open web investment if it offered agentic automation comparable to search and social, which reads less as an endorsement of the open web than as an account of why budgets sit elsewhere. Vendor-commissioned research of this kind carries an obvious interest.

Adjacent terms

Programmatic advertising is wider, covering any automated transaction including in-app, video and connected television. Open-web display is one environment within it.

Display network describes an ad network such as the Google Display Network, which is a single source of demand reachable within open-web display, not a synonym for the market.

Open internet is a vendor term that usually stretches beyond web pages to connected television, audio and digital out-of-home. Open-web display stops at the browser.

Remnant describes inventory status rather than environment. Unsold inventory exists inside walled gardens too; open-web display covers both reserved and auctioned impressions.

Recent developments

Remedies arrived on September 2, 2026. Brinkema declined to order divestiture of AdX or DFP and adopted behavioural obligations instead, in a two-page order pointing to a memorandum opinion sealed for fourteen days. The result rewrites the auction without changing its ownership, after Google had already removed Unified Pricing Rules from Ad Manager in December 2025.

Commercial repositioning has run alongside the litigation. Google retired the standalone Display campaign type, folding the Display Network into Demand Gen as an opt-in channel rather than a default. Taboola, meanwhile, took on display sales for NBC News in its first agreement beyond native, a premium publisher consolidating a declining format with a performance vendor. Traffic is the constraint underneath all of it: publishers reported impression declines around 40% in January 2026, and USA TODAY Co. lost 22 million monthly unique visitors in a single quarter.

Timeline

  • 2000: Google launches AdWords as a self-service platform for search advertising
  • June 18, 2003: AdSense launches, giving publishers access to a network of more than 100,000 advertisers
  • 2008: Google acquires DoubleClick for $3.1 billion, gaining DFP and the nascent AdX exchange
  • Early 2014: AdX wins 53% of the inventory DFP puts to auction
  • 2015: Header bidding achieves widespread adoption among large publishers
  • 2017: Project Poirot shades Display and Video 360 bids on non-AdX exchanges
  • 2018: Google launches Open Bidding as a server-side answer to header bidding
  • March 6, 2019: Google announces the move of Ad Manager inventory to a unified first price auction
  • 2019: Last Look is withdrawn and Unified Pricing Rules introduced, barring higher floors for AdX than for rival exchanges
  • January 24, 2023: The Department of Justice and eight states file the ad tech antitrust suit
  • April 17, 2025: The court finds Google monopolised the publisher ad server and ad exchange markets for open-web display
  • September 5, 2025: The European Commission fines Google 2.95 billion euros in Case AT.40670
  • November 21, 2025: Closing arguments conclude in the Virginia remedies phase
  • December 2025: Google removes Unified Pricing Rules from Ad Manager
  • January 14, 2026: The Commission publishes the provisional public version of its ad tech decision
  • April 16, 2026: IAB and PwC report United States internet advertising revenue of $294.6 billion for 2025
  • September 2, 2026: The court rejects divestiture and adopts behavioural remedies

Summary

Who. Web publishers that do not build their own ad tech, and the intermediaries they rent it from: publisher ad servers including Google Ad Manager, Equativ, Kevel and Microsoft; exchanges including AdX, Index Exchange, Magnite, OpenX, PubMatic and Kargo; and buy-side tools including Display and Video 360, The Trade Desk, Amazon DSP and Google Ads. Regulators in Washington and Brussels have made the same set of firms the subject of enforcement.

What. Display advertising served on websites that use third-party infrastructure to match advertisers to inventory, excluding walled gardens, search, social, in-app and instream video. The category is defined by who controls the plumbing rather than by the shape of the advertisement.

When. It emerged as direct selling gave way to networks in the late 1990s, took its present shape after the DoubleClick acquisition in 2008 and the arrival of header bidding in 2015, and became a formally recognised antitrust market with the April 2025 liability ruling.

Where. In the browser, on news sites, blogs, forums and specialist publications, transacted through ad servers, exchanges and demand-side platforms owned by neither buyer nor seller.

Why. It is the revenue model for the part of the web that has no platform behind it and no paywall in front of it. Its share of display buying has fallen from over 40% to 11% of AdWords impressions in six years, its supply is shrinking as search referrals recede, and the mechanics that determine who bids on it first have now been rewritten twice by courts on two continents.