Interop 2024: The New Browser Compatibility Targets

For the third consecutive year, browser vendors have joined forces under the Interop banner to close compatibility gaps and push the web platform toward a more unified state. Interop 2024 builds on the groundwork laid by the 2022 and 2023 editions, this time with 12 fresh focus areas and five carryovers from last year that still need finishing touches.

The core idea behind Interop hasn't changed: it's a shared testing and benchmarking effort where each browser is scored against a common set of web standards tests. The goal is to eliminate the subtle—and not so subtle—differences in behavior that force developers to write workarounds and degrade the experience for end users.

The full list of focus areas for 2024 includes:

  • Accessibility
  • CSS Nesting
  • Custom Properties
  • Declarative Shadow DOM
  • font-size-adjust
  • HTTPS URLs for WebSocket
  • IndexedDB
  • Layout
  • Pointer and Mouse Events
  • Popover
  • Relative Color Syntax
  • requestVideoFrameCallback
  • Scrollbar Styling
  • @starting-style and transition-behavior
  • Text Directionality
  • text-wrap: balance
  • URL

The Interop 2024 Dashboard tracks scores for each browser and gives a detailed look at the specific tests and features involved. For Chrome, a few areas stand out as needing the most work to reach full marks.

Getting CSS Nesting Back on Track

The CSS nesting module puts native nesting in the browser, removing the need for preprocessors to achieve that structure. Stylesheets become more readable and modular, and file sizes can shrink because the browser does the parsing itself.

All major browsers have shipped nesting, but spec changes introduced since initial implementations have created discrepancies. Interop 2024 aims to bring every engine in line with the current specification. Chrome developers can refresh their knowledge of the feature and review the relaxed syntax update that permits nesting bare element tag names.

Balancing Short Text Blocks

The text-wrap: balance value asks the browser to distribute lines evenly across a text block, typically headings or short passages, to avoid widows. While support exists, implementations differ across the various shorthand and longhand forms of the text-wrap property. The 2024 effort targets true interoperability for these.

Developers who want to understand the motivation can read about CSS text-wrap: balance, the argument for ending typographic widows on the web, and practical notes in this walkthrough.

WebSocket Over HTTP URLs

Until recently, the WebSocket constructor only accepted ws: and wss: schemes, which made relative URLs impossible and pushed developers into writing workaround code. The specification has since been revised to accept http: and https: schemes, which are normalized internally to ws: and wss:. Safari, Firefox, and Edge add support for http(s) schemes during Interop 2024.

Relative Color Syntax

The CSS Color 5 specification introduces Relative Color Syntax for manipulating colors in stylesheets—lightening, darkening, or desaturating them without a preprocessor. The feature is largely interoperable already, but support for the currentcolor keyword is a gap that the Interop 2024 tests cover. Developers can get a thorough primer on CSS relative color syntax to see the range of options.

Tracking Progress Through the Year

Score updates for both stable and experimental browser builds will appear on the public dashboard throughout the year, refreshed as new versions are tested. The idea is to make progress visible and hold the effort accountable. Announcing browser projects for Interop 2024 have detailed the focal points on their own pages:Apple, Bocoup, Igalia, Microsoft, and Mozilla.