CSS Cascade 5 (Layers)

Specification for Cascade Layers @ W3C CSS Working Group

September 2020 – May 2022

Cascade & Inheritance Level 5 defines Cascade Layers – allowing authors to define explicit contained layers of specificity.

The CSS Cascade is designed to balance concerns, and give some styles priority over others. That starts with a balance of power between three “origins” – users, authors, and user agents (aka browsers).

By default, author styles override user styles, which override user-agent styles. However, the order is reversed for any styles that are marked as !important – so that browsers can define what is out of bounds, and users can insist on their most essential preferences.

Site authors are often able to write styles without much consideration for the other origins involved. But as site styles have become more complex – relying on larger teams and third-party code – there are also many ‘concerns’ represented within the single origin. In order to balance those concerns, authors have been limited in their control of the cascade:

The binary !important flag can be applied to individual style declarations, with important declarations overriding non-important declarations.
The ‘selector specificity’ heuristic gives more narrowly targeted (specific) selectors priority over more broadly applied selectors.
Later declarations override earlier declarations.

Over the years authors have developed a number of different ‘conventions’ to help manage styles, especially in order to ‘tame’ or control the cascade. In many cases, these conventions rely on balancing concerns between defaults, basic typography, design systems, component libraries, themes, and third-party tools.

At the end of 2019 I suggested that authors should be able to define ‘custom origins’ within the existing cascade origins. The goal was to:

provide a more explicit tool for balancing concerns and resolving issues often seen as “specificity problems”
help teach the powerful concepts already built into the core of the language
make it more clear how !important is designed to work