Design — spatial composition

When to Use

Use this skill when you need design — spatial composition.

A model cannot trust its own UI output. Everything else follows from two failures.

1. It can't see what it made

UI is generated as a token stream, never as pixels — so the model cannot perceive collisions, overlap, imbalance, or broken spacing. It will write a headline that runs into the hero image and have no idea.

Render it and judge the image, not the code. Serve with any static server (e.g. python3 -m http.server or npx serve) and screenshot headless via Playwright. Screenshot at a few widths.

Critique with fresh eyes — not your own. Grading your own output rationalizes it; the builder looks at its overlapping headline and calls it fine (this is exactly how a real collision shipped in testing). Use a separate judge — a subagent that did not write the page — and tell it to hunt for what's wrong: collisions, edge tangents, ragged alignment, lopsided weight, no clear focal point, breaks at some width. Fix, re-render, re-judge.

2. Its first idea is the average

Whatever it produces first is the mean of its training data — and there is more than one mean:

• the generic-AI mean: Inter, purple-on-white gradients, centered single column, three equal cards;
• the designer-trend mean: oversized condensed caps, dark-mode + grain, monospace "vibes" microtext, sticker badges.

Landing on the second isn't taste — it's a more flattering average, which is why it slips past. Treat your first instinct as the mean and deviate deliberately — toward this product's specific world (use design-thinking's domain / color-world / signature as the direction), not toward another trend. If the result could be any startup, you shipped the mean.

3. So don't prescribe a style

Any fixed rule — a 12-col grid, an 8-point scale, "mono = data" — becomes next cycle's mean, and a blind model executes it into collisions anyway. Prescribe the process, not the look: see it with fresh eyes, and push off the average toward the domain. Taste supplies the direction (design-thinking / design-philosophy); this skill only insists you look and don't ship the mean.

For iterative spatial tuning, a local page with live controls (sliders, pickers, drag handles) beats one-shot critique.

4. NEVER ship horizontal overflow — THE mandatory gate, no exceptions

BLOCKING GATE. You may not call any web UI "done", "working", "fixed", or "looks good" until you have run the scrollWidth check below at a narrow width THIS turn and seen 0. Not "I added overflow-x:clip so it's fine." Not "it looked fine at my width." MEASURE. Narrow. Every time. If you didn't measure, it isn't done — say "haven't checked overflow yet" instead of claiming done.

A side-to-side scrollbar that doesn't match the content is the single most common and most embarrassing layout failure, and it ships over and over because the dev viewport is wide enough to hide it — the overflow only appears once the window is narrower than some element. It is invisible at desktop width, so the §1 render-critique loop will NOT catch it unless you screenshot narrow. Separate, explicit, non-negotiable gate.

It recurs because layouts GROW after they were last checked. Every time you add a nav tab, a toolbar button, a header control, a chip, a wider equation/<pre>, or any new item to a flex/inline row, you have invalidated the last overflow check — the row that fit yesterday now pushes past the edge between ~720–1200px while your 1440px dev window shows nothing wrong. (Real ship, 2026-06, TWICE: a progress-bar edge label overflowed 23px; then a flex-wrap:nowrap header that grew 4 tabs scrolled the whole page 309px across 720–1200px — both invisible at dev width, both caught only by measuring narrow.) So: any change that adds an element to a horizontal row re-arms this gate. Re-measure.

Default defenses to apply up front (so the gate passes by construction):

• Header / nav / toolbar rows: flex-wrap: wrap, never nowrap. A growing single-row flex is the #1 source of this bug. Wrapping is a no-op when it fits and saves you when it doesn't.
• body { overflow-x: clip } as a backstop on every app (clip, not hidden — keeps sticky/anchored layouts working). A backstop, NOT a substitute for measuring.

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