modern-css

Modern CSS

STOP. Before any CSS edit, audit, or review, output the detection one-liner to the user:

Target: {browser target}. Tokens: {explicit/implicit/none, with prefix if any}. Boundary: {shadow internals / light DOM / web-component consumer}.

This is non-negotiable. It gives the user an interrupt point — they can correct miscalibration before the first edit corrupts output. Skipping the one-liner is the most common way this skill fails. Violating the letter of this rule violates the spirit of this skill; there are no exceptions for "simple edits," "quick fixes," or "I already know this project."

How to fill it in: run the detection spine below (Step 1 browser target → Step 2 tokens → Step 3 boundary). What to do next: before the first CSS edit, (a) create one TodoWrite task per applicable reference (see routing table) and (b) Read each file to mark its task complete. Reading means the Read tool, not memory. A reference with no Todo is a reference that will not get read — this is how prior sessions of this skill have failed.

Write CSS that matches the project's real context — not generic 2018 CSS, not bleeding-edge features the target doesn't support, not raw values when the project has tokens.

How to use this skill

1. Run the detection spine (below) once per styling context.
2. Report what was detected — the one-liner is mandatory.
3. For each situation in the routing table that applies, create a TodoWrite task "Read modern-css references/{leaf}.md — {one-line why}" and complete it by Read-ing the file. Do this before the first CSS edit. See "Enforcing reference loading" below.
4. Follow the load-bearing rules at all times.

Detection spine

Run once per styling context (new file, new component, or context shift). Cache mentally for the session.

Step 1 — Browser target

Check signals in order. First match wins.

1. package.json has engines.vscode → VS Code extension path:
   • Has web entry point (browser field in package.json) → target is Baseline "widely available" (web surface = many browsers/versions).
   • Desktop-only → map the VS Code engine version to its Electron version, then to its Chromium version. Use that Chromium as the target.
2. .browserslistrc, browserslist, or package.json.browserslist → use the query literally.
3. Explicit note in README / CLAUDE.md about browser support → honor it.
4. None of the above → default to Baseline "widely available".

Output: a concrete target string, e.g. "Chromium 122+", "Baseline widely available", "last 2 Chrome + last 2 Firefox".

Step 2 — Token system

Detect explicit first. If nothing, infer implicit. Never propose formal tokenization unsolicited.

Explicit signals:

• --vscode-* tokens referenced anywhere → VS Code webview context. --vscode-* IS the semantic layer. Use it directly; don't wrap it.
• CSS files with --*: declarations → enumerate via grep.
• Token files: tokens.css, theme.css, variables.css, or a design-tokens/ directory.
• Tailwind config (tailwind.config.{js,ts,cjs,mjs}) → extract theme tokens.
• CSS-in-JS theme exports.

Convention inspection (when explicit tokens exist):

• Casing (kebab / camel / other)
• Prefix pattern (e.g., --gl-*)
• Tier structure (primitives only / primitive + semantic / three-tier)

Implicit pattern fallback (no explicit tokens):

• Grep existing CSS for actual values used: spacing (padding, margin, gap), sizing, typography (font-size, line-height), border-radius.
• Extract values that appear 3+ times → identify implicit scales (e.g., "4/8/12/16/24/32px — 8-point spacing").
• Match these when writing new CSS.

Output shape:

• Explicit: "Project tokens: --gl-* prefix, kebab-case, two-tier. Use --gl-space-md, not 16px."
• Implicit: "No explicit tokens. Implicit spacing: 4/8/12/16/24/32 (8-point). Match these."
• Neither: "No consistent patterns. Use reasonable round values."
• VS Code: always flag --vscode-* availability.

Step 3 — Styling boundary

Signals:

• customElements.define / extends HTMLElement in target file → shadow root context likely.
• Lit / Stencil / FAST imports in the file.
• Deps on @spectrum-web-components, shoelace, etc.
• <style> tag inside a lit template → shadow root internals.
• Plain .css file or global stylesheet → light DOM.
• Styling an existing custom element from outside → web component consumer.

Output — one label:

• Shadow root internals → use :host, styles are scoped automatically, expose ::part thoughtfully.
• Web component consumer → custom properties + ::part + ::slotted only; don't reach inside.
• Light DOM → standard cascade.

Report what was detected

Before writing CSS, output a one-liner:

Target: {target}. Tokens: {tokens}. Boundary: {boundary}.

This is non-negotiable. It surfaces miscalibration before it corrupts output. The user can interrupt and correct.

Ambiguity handling

Load-bearing rules (always apply)

• Never reach for !important or specificity escalation to fix a cascade problem. Use @layer or :where() instead. (→ references/cascade.md)
• Never invent CSS syntax. If unsure a property or value exists as specified, flag uncertainty rather than guessing. Verify via MDN or caniuse when tool access allows.
• Never use features above the detected browser target. If a modern feature is needed but unsupported, say so instead of silently downgrading.
• Never hardcode values when explicit or implicit tokens exist. Match the token system that's already there; don't impose a new one.
• Never use viewport media queries for component-internal layout. Use container queries instead. Pages still use media queries; preference/capability media queries (prefers-reduced-motion, hover, etc.) are always media queries regardless. (→ references/responsive.md)
• Never reach into shadow DOM from outside. Use ::part, ::slotted, or custom properties. (→ references/selectors.md)
• Never use transform: translateZ(0) or permanent will-change for layer promotion. Use isolation: isolate or contain instead. (→ references/performance.md)
• Never fix layering by escalating a raw z-index. A raw z-index climbing past ~100 is the signal to use a semantic tier token (--gl-z-* in GitLens webviews), isolation: isolate on the trapping ancestor, or the top layer (<dialog>.showModal() / [popover]) — not a bigger number. Shadow-isolated internals keep small raw ordinals (1/2/3). (→ references/performance.md)
• In GitLens webviews, never apply a --gl-shadow-* elevation token raw — go through the elevated-surface helper (Lit elevatedSurface / SCSS @include elevated-surface), which pairs the shadow with the border that survives forced-colors (where every shadow renders as nothing). Tokens, tiers, and the elevation model live in docs/webview-styling.md.
• Name custom properties by role, not appearance. --color-accent, not --blue. In component code, prefer semantic tokens over primitives. (→ references/theming.md)

When to load which reference

Read the relevant reference file before writing CSS in these cases:

If a task spans multiple situations (e.g., building a new component touches layout + theming), load all matching references.

Enforcing reference loading (TodoWrite gating)

Two prior field tests of this skill (n=1, n=2) both read zero references. Prose that says "read the reference before writing CSS" gets skipped. A pending TodoWrite task is harder to skip because it sits in the visible task list as incomplete.

Flow:

1. Run detection spine → output the one-liner.
2. Scan the routing table. For each row that applies to the current task, call TodoWrite to create a task formatted:

   Read modern-css references/{leaf}.md — {one-line why it applies}
   

3. Before the first CSS Read or Edit against project code, work through the tasks: use the Read tool on each referenced file, then mark the corresponding task completed. You may read in parallel.
4. If, after Read-ing, a reference turns out not to apply, mark the task completed with a one-sentence note in chat explaining what you expected vs. what the file covered. Never silently skip.

Rationalization check. If you catch yourself thinking "I already know what that reference says" — create the task and Read anyway. Skills evolve; your memory is stale. The discipline exists because the skill's authors have seen agents (including this one) confidently skip references and ship miscalibrated output. Memory is not evidence.

What "applies" means. An audit that touches cascade, tokens, animations, and selectors creates four tasks, not one. For audits specifically, assume cascade.md, theming.md, and the audit-relevant subset of selectors.md/responsive.md/animation.md/performance.md all apply unless you can name what makes them irrelevant.

When auditing existing CSS

When reviewing a codebase rather than writing new CSS:

• Weight recommendations to the deployment context. A VS Code extension that doesn't support RTL still benefits from logical properties (expressiveness), but a 290-instance migration has different priority than in an i18n-ready web app. State the context-specific rationale, not generic best-practice justification.
• Triage within findings. Don't treat all instances as equal severity. A continuous spinner without prefers-reduced-motion is higher severity than a 150ms opacity fade. A hardcoded color in a themed component is higher severity than one in a syntax-highlighting block. Report the severity distribution, not just the count.
• Recommend implementation strategy, not just the fix. "Create a shared mixin" vs. "wrap individually" vs. "global reset" are different approaches with different tradeoffs. Name the strategy.
• Classify hardcoded values before recommending token migration. Three categories: (a) should be a token (repeated semantic value), (b) should derive from a token via color-mix() or relative color syntax (opacity/lightness variant), (c) intentionally absolute (shadows, syntax highlighting, one-off decorative). Only (a) and (b) need action.
• Check for architectural gaps, not just lint-level issues. In web component codebases: ::part() exposure (too many? too few?), :host styling consistency, style duplication across components, animation performance (compositor-friendly transform/opacity vs. layout-triggering width/height/top/left), dead/unused CSS.
• Prioritize across findings. Rank by: (1) Broken under real conditions — hardcoded values that fail on theme switch, animations that ignore reduced-motion, z-index bugs from misunderstood stacking contexts. These are bugs. (2) Architectural debt that compounds — style duplication, inconsistent :host patterns, no ::part exposure strategy. Gets worse with every new component. (3) Modernization with measurable benefit — performance gains, fewer selectors, simpler state. Worth doing but not urgent. (4) Modernization for code quality — cleaner shorthands, modern selectors replacing working old ones. Apply to new code, migrate opportunistically. Lead with tier 1, end with tier 4.
• Specify migration scope for large findings. For findings with 50+ instances: recommend new-code-only unless the finding is severity tier 1 (broken under real conditions). For tier 1, recommend a focused migration. For tier 2-4, recommend a lint rule for new code + opportunistic migration when already touching the file. A small team cannot absorb a 290-instance migration alongside feature work.