The Architecture of CSS Support Queries

The provided text offers a technical deep dive into @supports, the CSS conditional rule used to perform feature queries for modern web development. It explains how the browser distinguishes between supported features, unsupported syntax, and invalid grammar to ensure forward compatibility and graceful degradation. The overview spans multiple specification levels, detailings how the technology has evolved from basic property-value pairs to complex selector queries and future Level 5 functional tests. Additionally, the text contrasts the implementation of these queries across stylesheets, JavaScript APIs, and @import rules, highlighting specific syntax requirements like parenthetical grouping. By examining the Blink engine's internal logic, the source illustrates how CSS maintains a robust, tri-state parsing model to prevent new code from breaking older environments.