Planning for the 2027 edition of the browser industry's Interop project is offering an early look at CSS features that could reach wider, more consistent support. The annual effort brings together major browser vendors and web-platform contributors to select technical areas where coordinated implementation and testing can reduce differences between browsers.
Google, Apple, Microsoft and Mozilla participate alongside engineering groups including Igalia and Bocoup. The process does not guarantee that every proposed feature will be selected, but inclusion can accelerate compatibility work. CSS Subgrid demonstrates the model: Firefox implemented it in 2019, adoption elsewhere remained limited for years, and its selection for Interop 2023 was followed by broad availability across modern browsers by the end of that year.
One proposal under consideration would bring author-defined functions to CSS. Using an `@function` rule, developers could package calculations and return reusable values without moving the logic into JavaScript. Web educator Josh Comeau highlighted coordinate conversion as one possible use, particularly for animation work that repeatedly translates between polar and Cartesian systems. The feature is already implemented in Chromium-based browsers, but broader availability would depend on work by other engines.
Another area focuses on the shape of element corners and borders. The proposed `corner-shape` property goes beyond the circular rounding supplied by `border-radius`, offering alternatives such as bevels and smoother superellipse-style corners. Because it changes the border box itself, outlines, borders and shadows can follow the resulting contour. That is different from approximating the effect with clipping or masks. Chromium supports the feature, while Firefox and Safari did not at the time of the October 2026 account.
A lower-level `border-shape` proposal would let authors describe custom outlines with straight lines, arcs and Bézier curves. The approach could support irregular cards, cutouts and animated transitions while preserving the connection between the element's shape and its visual border. Both shape proposals remain candidates rather than universal production features.
The next stage of CSS Anchor Positioning is also being discussed. The existing API lets a floating element, such as a tooltip, attach itself to another element and try fallback positions when space is limited. A Level 2 proposal would add a query that tells the stylesheet which fallback position was used. That information matters when a tooltip includes an arrow or decoration that must point back toward its trigger after the box flips from above to below.
A more preliminary idea is an `inherit()` function that would allow an inherited value to be modified with calculations. Standard CSS can copy a parent's value with the `inherit` keyword, but designers sometimes need an adjusted value—for example, a smaller inner corner radius nested inside a larger rounded container. The proposal has not yet been implemented in browsers.
These candidates illustrate the practical aim of Interop: not simply adding syntax, but aligning the behavior developers can depend on. Until the 2027 focus areas are finalized, the features carry different levels of support and should be treated as experiments or progressive enhancements. Their selection would signal where browser teams intend to concentrate compatibility work next.
