csslisted

# CSS in Monochromatic Fires when editing CSS files, writing component styles, picking between CSS approaches, reviewing CSS in a PR, choosing sizing units, deciding on a selector strategy, or adding inline styles to a component. Other surface phrases that should trigger the skill: "edit this CSS", "style this", "write a stylesheet", "add CSS to", "review CSS", "what's the right CSS for", "this component needs styles". The skill applies to `.css` files, CSS-in-JS, inline `style` attributes, and any styling work in the repo. Single source of truth for the browser baseline: see `PHILOSOPHY.browser-support.md` at the repo root. The skill encodes the platform-feature defaults, browser baseline, sizing rules, logical properties, shorthand rules, design-token colors, declaration discipline, accessibility minima, mixin shape, nesting depth, and state-attribute conventions. ## Browser baseline Firefox ESR 140 (June 2025). All CSS features must be supported by this baseline. Newer features that the baseline lacks need a feature query (`@supports`) and a fallback, or they need to be deferred. When in doubt about a feature, check the Firefox ESR 140 caniuse percentage before reaching for it. Anything supported in evergreen Firefox but not ESR 140 is off-limits without a feature query. ## Native platform features first Pick the native option before reaching for a library: - `<dialog>` for modals. - Popover API (`popover` attribute, `::backdrop` pseudo-elem