Designing Frontend Interfaces

Overview

Distinctive interfaces come from committing to one aesthetic direction and executing it with a locked system of values. Generic interfaces come from picking each value ad hoc at the moment it is needed.

Core principle: Decide the system before you write the components. Every number in the finished CSS should trace back to a scale you chose deliberately.

This skill covers visual craft. Two companion skills cover the rest of interface quality:

• REQUIRED SUB-SKILL for anything with flows, forms, or non-trivial state: use designing-user-experience
• REQUIRED SUB-SKILL before shipping: use building-accessible-interfaces
• To critique a finished UI, use reviewing-interface-quality

The Iron Law

NO COMPONENT CODE BEFORE THE DESIGN BRIEF AND TOKEN BLOCK EXIST

The brief is 8 lines. The token block is one CSS block. Together they cost about ninety seconds and they are what separates a designed interface from a decorated one.

Wrote components first? The values are already arbitrary. Extract them into a real scale before continuing — do not rationalize the numbers you happened to type.

Checklist

Create a task for each item and complete them in order:

1. Write the design brief — 8 lines, in your response, before any file is created
2. Choose an aesthetic direction — from references/aesthetic-directions.md, or invent one, and name it
3. Lock the token block — type scale, color ramp, space scale, radius, shadow, motion. One CSS block.
4. Build the layout skeleton — structure and spacing, no decoration yet
5. Build components — every interactive element gets all five states
6. Add the one memorable moment — the single thing someone will remember
7. Self-review against the rubric — see Self-Review below; fix what fails
8. Verify visually — render it and actually look at it

Step 1: The Design Brief

Write this out before creating any file. It is 8 lines of plain text in your response — not a document, not a file.

Purpose:    What job does this interface do?
Audience:   Who uses it, in what context, on what device?
Tone:       One named direction (see references/aesthetic-directions.md)
Reference:  A real-world visual analogue — a magazine, a era, a discipline
Palette:    Base / surface / ink / one accent — as hue intentions, not hex yet
Type:       Display face + text face, and why this pairing
Memorable:  The one thing someone will describe to a friend
Restraint:  What this design deliberately does NOT do

The Restraint line matters most. A design with no stated restraint becomes a design with every effect applied at once.

Worked example:

Purpose:    Changelog for a developer infrastructure tool
Audience:   Engineers skimming on a wide monitor, 30 seconds per visit
Tone:       Technical Broadsheet — newspaper density meets terminal precision
Reference:  Financial Times print edition, 1990s man pages
Palette:    Warm newsprint base, near-black ink, one signal red for breaking changes
Type:       Instrument Serif display / IBM Plex Mono text — editorial authority, machine detail
Memorable:  Version numbers set enormous in the left margin, hanging outside the text column
Restraint:  No cards. No shadows. No rounded corners. Rules and space only.

Step 2: Lock the Token Block

Every value the interface uses comes from here. If a number is not in this block, it does not belong in a component.

:root {
  /* TYPE — one ratio, applied consistently. 1.200 minor third (dense UI),
     1.250 major third (general), 1.333 perfect fourth (marketing),
     1.500 perfect fifth (editorial/poster). Pick ONE. */
  --font-display: "Instrument Serif", Georgia, serif;
  --font-text:    "IBM Plex Mono", ui-monospace, monospace;
  --step--1: 0.833rem;  --step-0: 1rem;     --step-1: 1.25rem;
  --step-2:  1.563rem;  --step-3: 1.953rem; --step-4: 2.441rem;
  --step-5:  3.052rem;  --step-6: 3.815rem;

  /* SPACE — one base unit and its multiples. Never a value in between. */
  --space-3xs: 0.25rem; --space-2xs: 0.5rem; --space-xs: 0.75rem;
  --space-s:   1rem;    --space-m:   1.5rem; --space-l:  2rem;
  --space-xl:  3rem;    --space-2xl: 4.5rem; --space-3xl: 7rem;

  /* COLOR — a ramp, not a set of one-offs. One accent, used sparingly. */
  --base:    #f4f1ea;   /* page */
  --surface: #ffffff;   /* raised */
  --ink:     #12100e;   /* primary text */
  --ink-2:   #55504a;   /* secondary text */
  --ink-3:   #8b857c;   /* tertiary / meta */
  --line:    #ddd7cc;   /* borders, rules */
  --accent:  #c1362f;   /* <10% of visible surface */

  /* FORM */
  --radius: 0;          /* commit: 0, or 2-4px, or a full 999px pill. Not "some of each". */
  --border: 1px solid var(--line);
  --shadow: none;       /* if using shadows, define 2 — near and far. Never 5. */

  /* MOTION */
  --dur-fast: 120ms; --dur: 220ms; --dur-slow: 420ms;
  --ease:     cubic-bezier(0.2, 0, 0, 1);
  --ease-out: cubic-bezier(0.16, 1, 0.3, 1);
}

Rules for the block:

Step 3: Hierarchy

Vary one dimension at a time. Amateur hierarchy changes size and weight and color and spacing simultaneously; everything shouts and nothing leads.

Space groups more reliably than lines or boxes. Before adding a border or a card, try doubling the gap between groups and halving it within them. Most "needs a card" instincts are actually "needs correct spacing."

Step 4: Anti-Slop

Generic AI interface output has a specific, enumerable signature. These are not stylistic preferences — they are the exact defaults that make output identifiable on sight.

The convergence trap: across separate generations, do not keep reaching for the same "safe distinctive" choices. Space Grotesk, a dark slate background, and a lime-green accent is now its own cliché. Vary light and dark, serif and sans and mono, dense and airy, across designs. Check references/aesthetic-directions.md and pick one you did not pick last time.

Rationalization table:

Step 5: Component States

Every interactive element gets all five. Missing states are the most common gap in generated UI.

.btn              { /* rest */ }
.btn:hover        { /* pointer only — must not be the sole affordance */ }
.btn:active       { /* pressed — a real transform, not just a color shift */ }
.btn:focus-visible{ outline: 2px solid var(--accent); outline-offset: 2px; }
.btn:disabled     { /* visibly inert, and still readable */ }
.btn[aria-busy]   { /* loading — reserve the space, don't collapse the layout */ }

Never remove focus outlines. If the default outline is ugly, restyle it — do not set outline: none. See building-accessible-interfaces.

Step 6: Verify Visually

Rendering code and reading it back is not verification. Look at the actual pixels.

# Static HTML
python3 -c "
from playwright.sync_api import sync_playwright
with sync_playwright() as p:
    b = p.chromium.launch()
    pg = b.new_page(viewport={'width':1440,'height':900})
    pg.goto('file:///abs/path/index.html'); pg.wait_for_timeout(600)
    pg.screenshot(path='/tmp/desktop.png', full_page=True)
    pg.set_viewport_size({'width':390,'height':844})
    pg.screenshot(path='/tmp/mobile.png', full_page=True)
    b.close()"

Then read both PNGs and check them against the rubric. For dev servers, use the testing-webapps skill's with_server.py helper.

What to look for that only shows in the render: text overflowing containers, collapsed layouts at 390px, invisible low-contrast text, elements touching viewport edges, a heading that wraps to one orphaned word, images without dimensions causing layout shift.

Self-Review

Score each line honestly. Any no means fix it, not explain it.

• Every spacing value in the CSS is a token from the block
• The type scale is one ratio; no size was chosen by eye
• Exactly one accent color, under 10% of visible surface
• One radius value throughout
• A stranger could name the aesthetic direction from a screenshot
• The Memorable line from the brief is actually present in the build
• The Restraint line was actually respected
• Nothing from the Anti-Slop table appears
• All five states exist on every interactive element
• Focus is visible and never suppressed
• Verified at 1440px and 390px by looking at screenshots
• Body text passes 4.5:1; large text passes 3:1

Reference Files

Load these when you reach the relevant decision — not upfront.

Common Mistakes

Tokens declared, then ignored. The :root block exists but components use padding: 14px. Grep your CSS for raw px values before finishing.

Distinctive typeface, default everything else. Swapping Inter for a display face while keeping the card grid, the uniform radius, and the gray body text produces generic output in a costume.

Maximalism as an excuse for incoherence. Maximalist directions still need one type scale, one space scale, one palette. More elements, same system.

The brief describes a design the build does not deliver. Re-read the brief at Step 7. If Restraint: no shadows and the build has shadows, the build is wrong.

Complexity mismatched to the vision. A refined-minimal direction implemented with elaborate scroll animations reads as confused. Match implementation effort to the stated tone.