Agent Skills: Crafting pure-CSS Tailwind plugins

Crafting pure-CSS Tailwind plugins

A great CSS plugin is a small, sharp tool: one idea, expressed as a family of composable utilities, that produces an effect with more depth than its size suggests — and zero JavaScript. This skill is how you get there. It pairs deep research into what CSS can actually do with a bespoke workbench for tuning the effect until it feels inevitable, then hardens and packages it for other people to depend on.

The bar is set by two reference plugins. Read their source before you start:

• tw-fade (~/Code/tw-fade/src/tw-fade.css) — scroll-edge fade masking. @property-registered scalars driven by scroll-driven animations, four edge gradients composed with mask-composite: intersect, a 14–17-stop sine-eased falloff, full RTL/logical correctness, gated behind @supports with a static fallback.
• tw-shimmer (~/Code/aui/assistant-ui/packages/tw-shimmer/src/index.css) — text & skeleton shimmer. Trig (tan()) to derive gradient geometry from an angle, a 17-stop sine-eased highlight, OKLCH relative color (oklch(from currentColor …)) + color-mix, container queries, a Firefox fix, and copious knob utilities via @utility + --value().

Neither uses a line of JavaScript. That is the standard.

When to use this skill

• Creating a new Tailwind v4 utility plugin or CSS effect from scratch.
• Pushing an existing effect from "fine" to "remarkable" — more depth, smoother motion, better edge-case handling.
• Packaging a CSS effect as an installable, hardened npm plugin.

When not to: if the effect genuinely needs runtime measurement, pointer tracking, or per-frame JS state, this skill's zero-JS constraint is the wrong fit — say so and reach for a JS approach. If a stock Tailwind utility already does it, don't wrap it.

The spine: one core utility, many knobs

Small "knob" utilities each set a custom property. One "core" utility reads those properties and renders the effect. Register the animated properties with @property.

This single pattern is the architecture of every plugin here. It makes effects composable (knobs stack in any order), overridable (arbitrary values and ancestor variables both work), and cheap (the core math is declared once).

@property --glow-angle { syntax: "<angle>"; inherits: false; initial-value: 0deg; }

@utility glow {                 /* CORE — consumes the props, renders the effect */
  --_hue: var(--glow-hue, oklch(0.72 0.18 255));
  background: conic-gradient(from var(--glow-angle), …var(--_hue)… );
  animation: glow-spin var(--glow-speed, 14s) linear infinite;
}
@utility glow-hue-*   { --glow-hue:   --value(--color, [color]); }  /* KNOB */
@utility glow-speed-* { --glow-speed: calc(--value(integer) * 1s); } /* KNOB */

See examples/starter-plugin.css for a complete, annotated worked example (tw-aurora), and references/tailwind-v4-authoring.md for the full @utility / @theme / --value() mechanics.

What "surprising depth and craft" actually means

These are the signatures that separate a remarkable plugin from a generic one. Treat them as the craft checklist:

1. Perceptually-eased gradients, not 2-stop ramps. A naive linear-gradient(black, transparent) bands and looks cheap. Both reference plugins use 14–17 stops following a sine/ease curve so the falloff reads as light, not a CSS default. This is the single biggest tell of craft.
2. @property-registered animatable customs. Registration is what lets a single keyframe smoothly interpolate a whole gradient, angle, or number. Unregistered customs snap at 50%.
3. OKLCH / relative color. Mix and derive colors in a perceptual space (color-mix(in oklch, …), oklch(from currentColor l c h / …)) so highlights and tints stay even across hues and adapt to currentColor.
4. Effects expressed as math over custom properties — the spine above — so the public API is just "set these props."
5. Pure CSS with honest fallbacks. Author the always-works version first, then layer the enhancement behind @supports. Never ship an effect that blanks out on a browser that lacks one feature.
6. Logical-property / RTL correctness, reduced-motion, and forced-colors awareness baked in from the start, not bolted on.
7. Reach for the esoteric. SVG filter primitives (feTurbulence grain, feDisplacementMap refraction, feColorMatrix duotone, gooey via blur + alpha-contrast), mask-composite, conic/@property-animated gradients, scroll/view timelines, style container queries. The depth comes from combining primitives most people never open. The catalog is references/css-arsenal.md.

The workflow

Work the phases in order. Don't skip the workbench — it's where the craft is won.

1 — Conceive & scope

Name the effect and its one job. Decide the public surface: the core utility and the 3–8 knobs that matter (speed, size, color, angle, intensity…). Sanity-check the zero-JS constraint holds. Write the one-line README description now — if you can't, the scope is fuzzy.

2 — Research the CSS surface

Open references/css-arsenal.md and find the primitives and recipes for your effect family (grain, glow, duotone, glass, distortion, fade, shimmer, …). For anything you're unsure is current, verify Baseline/support on the web (MDN, web.dev Baseline, caniuse) — support moves fast. Note which features need an @supports gate. For a genuinely novel effect, run a focused research pass (the deep-research skill) before committing to an approach.

3 — Spike the core effect

Get the effect working once, hard-coded, on a single element — in a scratch HTML file or straight in the workbench's <style>. Prove the central trick (the mask composite, the displacement map, the eased gradient) before generalizing it into utilities. Don't build the plugin API around an effect you haven't seen render.

4 — Build the workbench

Stand up the bespoke tuning lab (references/workbench.md; copy examples/workbench.html as the skeleton). Wire every knob to a slider, render the effect across a specimen matrix (many element types, sizes, surfaces, LTR/RTL, light/dark, nested), and add a lock + progress control to freeze-frame any animation for scrubbing. This is the instrument that turns guessing into seeing.

5 — Iterate to craft

Tune in the workbench against the matrix until it holds up everywhere, not just the hero. Push the gradient to enough eased stops to kill banding. Check the bad frames (lock + scrub). Tune over a patterned background to expose halos. Apply the craft checklist above. Export the dialed-in values.

6 — Harden & package

Bake the tuned values into the @utility/@theme source. Add @supports fallbacks, prefers-reduced-motion, forced-colors behavior, and RTL. Then package it: package.json style/exports, the framework-free dist build, verification, and a README with a utility table. Full recipe in references/packaging-and-hardening.md.

Decision shortcuts

• Effect won't animate smoothly? The driving custom property isn't @property-registered, or its syntax is typed and rejecting a fallback.
• Gradient looks banded/cheap? Too few stops, or linear instead of eased, or mixing in sRGB instead of OKLCH.
• Effect leaks into nested instances? Set inherits: false on the config @property registrations.
• Need a value the user passes both as a token and a raw number? Stack two --value() declarations (see authoring reference).
• Effect is expensive on a list? It's probably a filter, backdrop-filter, or an animated many-stop gradient re-rasterizing per frame — profile and confine it.

Files in this skill

• references/css-arsenal.md — the catalog of pure-CSS + SVG techniques and ready-made recipes, organized by effect family. Your edge.
• references/tailwind-v4-authoring.md — @utility, @theme, --value(), --modifier(), @variant, @property; the CSS-first plugin API.
• references/workbench.md — how to build the bespoke tuning lab.
• references/packaging-and-hardening.md — fallbacks, a11y, RTL, performance, package.json, the dist build, and verification.
• examples/starter-plugin.css — a complete, annotated tw-aurora plugin showing the full architecture end to end.
• examples/workbench.html — a self-contained, framework-free workbench wired to the starter, ready to copy and re-point.