design-system – skill Thiết kế & UI cho agent AI

Design system

When to Use

Use this skill when you need mechanical implementation invariants for frontend design: token architecture, typography hierarchy, loading order, FOUT prevention, chrome stability, motion timing, color semantics. Use with design when building components, pages, or design systems. Aesthetic direction is separate from these implementation guidelines.

Apply with design when implementing UI: components, pages, or design systems. Every color, type, and motion choice should trace back to these rules.

Token architecture

All colors map to a small set of primitives. No random hex values.

• Foreground: Text hierarchy (primary, secondary, muted).
• Background: Surface elevation (base, raised, overlay).
• Border: Separation hierarchy (subtle, default, emphasis).
• Brand: Identity and primary accent.
• Semantic: Destructive, warning, success (and optional info).

Use tokens in code (CSS variables, theme objects); never hardcode hex for UI.

Typography

• Hierarchy: Headlines — heavier weight, tighter letter-spacing for presence. Body — comfortable weight for readability. Labels/UI — medium weight, works at smaller sizes. Data — monospace, tabular-nums for alignment.
• Combine size, weight, and letter-spacing so hierarchy is clear at a glance. If you squint and can't tell headline from body, hierarchy is too weak.
• Fonts: pair a display font with a body font; keep hierarchy legible at a glance. *Which* fonts is direction, not mechanics — pick from the domain and push off your first/default instinct (the mean); see design-spatial §2.
• Data (functional only): real aligned numbers, IDs, timestamps in monospace with tabular-nums — mono earns its place when values line up in a column. Do NOT sprinkle mono on decorative eyebrow/metadata microtext ("35MM · DEVELOP · SCAN", fake spec captions) for a "technical" look — that's the current trend-slop, not data. See design-spatial §2.

Loading order — first seen, first loaded

The first viewport must paint complete and correct, fast. Order every resource by whether the user sees it first; the rest waits.

• Prioritize only the above-the-fold set (hero text, hero image/video, brand mark). Preloading everything is the same as preloading nothing — the true criticals lose the bandwidth race. Pick the few things in the first screenful and prioritize *those*.
• Fonts: self-host WOFF2. Convert OTF/TTF → WOFF2 (Brotli; ~half the bytes, identical glyphs) and <link rel="preload" as="font" type="font/woff2" crossorigin> the weights used in the first viewport. Never a render-blocking third-party font stylesheet — a Google Fonts <link> adds a CSS round-trip plus extra DNS/TLS before the font even starts downloading; self-host instead.
• LCP image/video: fetchpriority="high" on the hero image (or the video poster); <link rel="preload" as="image"> it when it's CSS-referenced (the parser can't see CSS url()s early). The hero box must never be empty — ship a poster/low-res placeholder so there's no blank frame.
• Below the fold: loading="lazy" decoding="async" on images; preload="none" (or "metadata") on video; defer non-critical JS. Always reserve space (aspect-ratio, or width+height) so deferred media can't shift layout (CLS).
• Keep the render-blocking head minimal: inline critical CSS, defer the rest.

Font loading — readable content and stable metrics

font-display: swap is the pop — it paints a fallback face, then swaps to the webfont and reflows. Do not use it for any text the user watches load (titles, wordmarks, hero copy). Balance visual stability with immediate readability; do not hide essential headings or controls while awaiting a font.

• For nonessential decorative display text only, optionally gate visibility on the real font. Synchronously in <head>, add a fonts-pending class to <html> that holds the display-font text at opacity: 0. On document.fonts.ready — kick it with document.fonts.load('<weight> 1em "Family"') for each critical face — swap to fonts-ready and fade the text in (~0.5s). Always include a safety timeout (~2.5s) that reveals regardless, so a font failure can never leave text permanently hidden.
• Pair this with preload + WOFF2 (above) so the hidden window is a few hundred ms, not seconds — the fade reads as intentional, not as a stall.
• For body text where a sub-perceptual swap is tolerable, at minimum kill the reflow: define a fallback @font-face (or font-family fallback) tuned with size-adjust / ascent-override / descent-override so the fallback occupies the same metrics as the webfont and the swap shifts nothing.

Worked example — an AR product-research page: a head script toggles fonts-pending → fonts-ready (titles fade in on fonts.ready, 2.5s fallback), preloads the four above-the-fold WOFF2 weights, and self-hosts the brand face so there's no Google round-trip.

Slow-loading content — never show the ugly intermediate state

Anything that *could* take a noticeable moment to be ready — fonts (above), large images, video, <canvas> scenes, Three.js / WebGL, lazy-loaded React islands, anything that fetches over the network or runs heavy main-thread setup — must either arrive fast or load gracefully. The default browser behavior (blank box → partial paint → reflow → final state) is the ugly intermediate state. Catch it.

Two levers; use both:

• Arrive faster. Compress (WOFF2 for fonts, Draco for glTF, WebP/AVIF for images, h264/h265 for video with preload="metadata"). Preload the *few* assets the first viewport actually needs (<link rel="preload">). Lazy-load below-the-fold so the LCP set isn't competing. Reserve the box (aspect-ratio, width+height) so deferred content can't trigger CLS.
• Load gracefully. Hide the in-flight state behind a styled placeholder, then fade the real thing in. Skeleton boxes, low-res blurred posters, a single ASCII glyph, even just the container's bg color — anything coherent with the design beats the default partial-paint.

What "ugly" looks like, concretely, and the fix:

| Symptom | Fix |

|---|---|

| Annotation labels stack at translate(0,0) (top-left of container) until JS positions them | Start labels at opacity: 0 with a transition: opacity ~0.35s; first projection sets inline opacity → CSS fades them up. |

| Canvas/WebGL paints empty/black for a frame on first render | Show a placeholder (CSS art, low-res poster image, or paper/skeleton fill) in the same box; remove it once the first real frame has rendered. |

| Lazy image fetches and snaps in with a layout-jump | aspect-ratio + <link rel="preload"> (above-the-fold) or loading="lazy" decoding="async" (below); fade from opacity:0 on the load event for the first paint. |

| Video poster pops to first frame on play | poster matches a still you control; once playing event fires, you've already had a clean handoff. |

| 3D model "appears" mid-screen with no transition | Keep the canvas visible but at opacity: 0; toggle a .viewer-ready class (or set inline opacity) inside the GLTFLoader success callback, after the first tick(). |

| Lazy React island flashes a fallback that looks worse than no UI | Replace Suspense fallback with a skeleton that traces the final layout, not a spinner. |

Rule of thumb: if a user could screenshot the page mid-load and you'd be embarrassed, you owe it a graceful state. The placeholder doesn't have to be fancy — it has to be *intentional*, sized correctly, and in the design language of what's coming.

Chrome stays still — status text never resizes layout

Persistent chrome (headers, nav, toolbars, search bars, status regions) must hold a constant height no matter what text lands in it. Transient status / loading / explanatory copy — "loading model…", "N matching · M indexed", empty-state hints — must not wrap to a second line and shove adjacent controls down. A status region that grows and shrinks as its message changes is a layout-jank bug, not dynamic content.

• For optional status summaries only, constrain to one line: white-space: nowrap; overflow: hidden; text-overflow: ellipsis so the longest message truncates instead of wrapping.
• Reserve the space up front: give the container a fixed height (or min-height) sized for the message, so the shortest and longest states — and the empty state — occupy the same footprint.

Only the content area should move while chrome stays fixed; layout shift from transient text reads as broken polish. (Concrete failure this prevents: in a search app, a model-loading message wrapping to two lines and pushing the search bar downward.)

Motion

• Keep timing consistent and purposeful; one well-orchestrated moment (staggered page load with animation-delay) beats scattered micro-interactions. Prefer CSS-only for HTML; Motion library for React. (Honor prefers-reduced-motion for public/multi-user proje

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