In March 2026, Mozilla published a post advocating for browser engine diversity. It described a situation that can only move in one direction.

Five browser engines powered the web in mid-2013: Trident, Gecko, WebKit, Presto, and Blink, which Google had just forked from WebKit that April. Today there are three. Blink holds close to 80% of the market. The two engines that vanished are staying gone, and the reason they're staying gone is the same reason they vanished.

Opera's Presto is the clearest case. Every quirky website that didn't render correctly required a custom workaround. Every new web specification required implementation from scratch. Since only Opera used Presto, web developers had little reason to test against it, which meant more rendering quirks, which meant more workarounds. By July 2013, Presto was gone. Five years later, Microsoft reached the same conclusion and rebuilt Edge on Chromium, abandoning its own EdgeHTML engine. Microsoft, a company with essentially unlimited engineering resources. The stated reasoning echoed Opera's: the specification environment moved too fast for a solo effort.

The mechanism underneath both decisions is the same, and it compounds. The Chromium codebase has grown to an estimated 20 to 40 million lines of code, roughly double what it was a decade ago. HTML became a living specification in 2011, meaning it never stops expanding. Every new API, every security requirement, every capability adds to the minimum viable implementation. The floor rises. And each time an engine dies, its users migrate to Chromium, which increases developers' incentive to optimize for Chromium, which makes the remaining engines' compatibility burden heavier. The mechanism feeds itself.

Rendering approaches shift between server and client. Programming paradigms cycle. Browser engine diversity clicks in one direction. Opera's Presto is gone. Microsoft's EdgeHTML is gone. The specification surface that proved too expensive for Microsoft has only grown since. Building a new engine from scratch today would mean implementing a larger surface area than the one that broke Microsoft's resolve.

Gecko, Mozilla's engine, sits at the center of a structural irony. Mozilla earned 86% of its 2024 revenue from Google, payment for making Google the default search engine in Firefox. The DOJ's antitrust remedies could disrupt the deal. Mozilla's CFO has acknowledged that without it, revenue would drop "precipitously." The last independent engine is financially tethered to the company whose browser dominance it was supposed to counterbalance.

Mozilla laid off 5% of its staff in February 2024. The Mozilla Foundation cut 30% that November.

Nobody publishes how many engineers work specifically on Gecko.

The direction is legible, though. The weight behind the door increases every year, and nobody on the other side is getting stronger.