January 2026 Baseline update: New CSS units, service worker modules, and multi-keyword display

The January 2026 edition of the Baseline digest covers the web platform features that recently crossed into the Newly available and Widely available categories. This month's batch includes several new root-relative CSS units, support for JavaScript modules in service workers, and a long-awaited syntax change for layout properties.

Newly available in Baseline

Several features moved into this category in January, including a new pseudo-class for view transitions and four typography-focused CSS units.

:active-view-transition is a CSS pseudo-class that targets the document's root element while a view transition is running. You can use it to apply global styling that only matters during the transition, such as adjusting the overlay background or tweaking z-index values for visual layering.

Service workers now support JavaScript modules across all major engines. By passing type: 'module' to navigator.serviceWorker.register(), you can use import and export statements inside the worker script, making it easier to organize code and share logic with the main thread.

The Navigation API offers a dedicated alternative to the History API for single-page applications. It centralizes intercepting and managing navigation events, including those from browser back and forward buttons. Features like the Maps event reduce the boilerplate needed for client-side routing.

Four new CSS length units are now available, each tied to properties of the root element's font:

  • rcap equals the cap height, the nominal height of capital letters, of the root font. It supports typographic scales based on the actual shape of the primary typeface.
  • rch is the root-relative equivalent of ch, representing the advance width of the "0" glyph in the root font. It is useful when a layout must precisely fit a set number of characters.
  • rex matches the x-height of the root font and helps with vertical alignment relative to lowercase letters.
  • ric corresponds to the ideographic advance measure (width or height of a CJK character) in the root font, which is important for layouts in Chinese, Japanese, or Korean.

Widely available in Baseline

January also pushed the multi-keyword syntax for the CSS display property to Widely available. Instead of the composed inline-flex value, you can now write display: inline flex, explicitly separating the outer (block/inline flow) and inner (flex/grid layout) behaviors of a box. This makes the layout model less opaque.

The animation-composition property also reached Widely available. It defines how competing animations combine when they affect the same property, with the replace, add, and accumulate keywords offering fine-grained control over layered effects.

JavaScript developers get safer array manipulation via the new by-copy methods: toReversed(), toSorted(), and toSpliced() return modified copies instead of changing the original array, making functional-style code less error-prone.

If you notice anything missing from Baseline coverage, file an issue in the tracker and it may be included in a future digest.