Inside the Interop Project: Boosting Safari Compatibility

This article explores the annual Interop project, a collaborative initiative where Apple engineers work directly with peers from Google, Mozilla, and Microsoft to standardize web technologies. It examines how this cross-industry partnership identifies key developer pain points, utilizes shared testing benchmarks, and ultimately enhances cross-browser compatibility for the Safari web browser.

What is the Interop Project?

The Interop project is an annual, cross-industry benchmark and coalition launched in 2022 by the major entities behind the world’s most popular web browsers: Apple, Google, Microsoft, and Mozilla, alongside software consultancies Bocoup and Igalia.

The primary goal of Interop is to eliminate inconsistencies between different browser engines—specifically Apple’s WebKit (Safari), Google and Microsoft’s Blink (Chrome and Edge), and Mozilla’s Gecko (Firefox). Before Interop, web developers frequently had to write extra code and workarounds to ensure a website looked and functioned the same way on Safari as it did on Chrome or Firefox. Interop solves this by agreeing on a set of priority web technologies each year and working until all participating browsers pass the exact same automated tests for those technologies.

How the Browser Vendors Collaborate

Despite being fierce competitors in the broader tech market, Apple, Google, Mozilla, and Microsoft follow a strict, cooperative workflow for the Interop project:

  • Gathering Developer Feedback: Each year, the coalition reviews survey data, such as the "State of CSS" and "State of HTML" reports, and opens a public proposal process. Web developers submit the browser bugs and missing features that frustrate them the most.
  • Selecting Focus Areas: Engineers from all four companies meet to review the proposals. They mutually agree on a list of 15 to 25 "Focus Areas" for the year. These have historically included CSS Grid, Flexbox, typography, color spaces, and form controls.
  • Writing Web Platform Tests (WPT): Once the features are chosen, engineers collaboratively write and refine tests in the shared Web Platform Tests repository. If a test reveals that a web standard is ambiguous, the engineers work together within standards bodies like the W3C and WHATWG to rewrite the specification so everyone interprets it the same way.
  • Public Scoring: Progress is tracked publicly on a live dashboard. Each browser is given a score from 0 to 100 based on how many tests it passes in the chosen focus areas.

Enhancing the Safari Web Browser

Apple’s participation in the Interop project directly shapes the engineering roadmap for WebKit, the engine that powers Safari across iOS, iPadOS, macOS, and visionOS.

Historically, developers often criticized Safari for lagging behind Chrome and Firefox in adopting new web standards or fixing long-standing layout bugs. Through the Interop project, Apple engineers have systematically addressed these complaints.

1. Resolving Ambiguities in WebKit

When Safari fails an Interop test, Apple engineers investigate the WebKit source code to see why it behaves differently from Blink or Gecko. Sometimes, Apple discovers a bug in WebKit and fixes it. Other times, Apple engineers prove that Safari is actually following the official web standard correctly, and it is Chrome or Firefox that needs to change. This back-and-forth ensures that fixes are based on consensus rather than one company's dominance.

2. Rapid Feature Deployment

Because the Interop dashboard is public, there is healthy competitive pressure for all browsers to reach a score of 100 by the end of the year. To keep Safari’s score high, Apple engineers rapidly implement agreed-upon features—such as CSS Container Queries, the :has() selector, CSS Subgrid, and the Popover API—and push these updates out to users via regular macOS and iOS software updates.

3. Testing in Safari Technology Preview

Apple uses the Safari Technology Preview, an experimental version of its browser updated roughly every two weeks, as the proving ground for Interop fixes. As Apple engineers write code to pass Interop tests, those changes are immediately deployed to the Technology Preview. The public Interop dashboard tracks this preview browser, allowing Google, Mozilla, and Microsoft engineers to verify Apple's progress in real time before the features ever reach the standard version of Safari.

The Result of the Partnership

By aligning Apple’s WebKit team with the engineering teams at Google, Mozilla, and Microsoft, the Interop project has drastically reduced the gap between Safari and its competitors. In recent Interop cycles, Safari, Chrome, Edge, and Firefox have all finished the year with scores well above 95% in the targeted focus areas. For web developers, this means less time debugging Safari-specific issues, and for Safari users, it results in a faster, more reliable, and visually consistent web experience.