Color contrast linter

src/editor/Editor.tsx

@@ -7,12 +7,14 @@ import { memo, useCallback, useEffect, useMemo, useRef, useState } from 'react';
 import { ParseError, parseDeck } from '@/ir/parse';
 import { planDeck } from '@/ir/plan';
 import { reorderSlide } from '@/ir/source-edit';
+import { lintColors } from '@/render/lint';
+import { resolveTheme } from '@/render/theme-resolver';
 import type { Brand, Deck, Density, Mode, ThemeRef } from '@/ir/schema';
 import { createAsset, assetSrc } from '@/storage/asset-store';
 import { getDeck, type StoredDeck, updateDeck } from '@/storage/deck-store';
 import { DeckRenderer } from '@/render/DeckRenderer';
 import { ExportPdf } from '@/render/ExportPdf';
-import { allPalettes, allStyles } from '@/themes/registry';
+import { allPalettes, allStyles, getPalette, getStyle } from '@/themes/registry';
 
 import { AssetsDrawer } from './AssetsDrawer';
 import { InsertMenu } from './InsertMenu';
@@ -150,6 +152,17 @@ export function Editor({ deckId }: Props) {
     [state.styleId, state.paletteId, state.density, state.mode],
   );
 
+  const lintWarnings = useMemo(() => {
+    try {
+      const style = getStyle(state.styleId);
+      const palette = getPalette(state.paletteId);
+      const resolved = resolveTheme(theme, style, palette, state.brand);
+      return lintColors(resolved.colors);
+    } catch {
+      return [];
+    }
+  }, [theme, state.styleId, state.paletteId, state.brand]);
+
   const result = useMemo(() => {
     try {
       const parsed = parseDeck(source, { theme, brand: state.brand });
@@ -316,6 +329,18 @@ export function Editor({ deckId }: Props) {
         />
 
         <div className="editor__preview-pane" onDrop={onPreviewDrop} onDragOver={onPreviewDragOver}>
+          {lintWarnings.length > 0 ? (
+            <div className="editor__lint" role="status">
+              <span className="editor__lint-icon" aria-hidden>
+                ⚠
+              </span>
+              <span className="editor__lint-text">
+                {lintWarnings.length === 1
+                  ? `${lintWarnings[0].label} contrast is ${lintWarnings[0].ratio} (needs ${lintWarnings[0].needs})`
+                  : `${lintWarnings.length} contrast issues — adjust palette or brand colors`}
+              </span>
+            </div>
+          ) : null}
           {result.ok ? (
             <PreviewStage
               deck={result.deck}

src/editor/editor.css

@@ -1425,6 +1425,34 @@
 /* Print
    --------------------------------------------------------------------------*/
 
+/* ─── Lint warnings strip ─────────────────────────────────────────────── */
+
+.editor__lint {
+  position: absolute;
+  top: 12px;
+  left: 50%;
+  transform: translateX(-50%);
+  display: flex;
+  align-items: center;
+  gap: 8px;
+  padding: 6px 14px;
+  background: rgba(217, 119, 6, 0.15);
+  color: #f59e0b;
+  border: 1px solid rgba(217, 119, 6, 0.4);
+  border-radius: 999px;
+  font-size: 12px;
+  font-weight: 500;
+  letter-spacing: 0.01em;
+  z-index: 10;
+  pointer-events: none;
+  backdrop-filter: blur(8px);
+}
+
+.editor__lint-icon {
+  font-size: 13px;
+  line-height: 1;
+}
+
 /* ─── Assets drawer ─────────────────────────────────────────────────────── */
 
 .assets-drawer {

src/render/lint.ts

@@ -0,0 +1,67 @@
+import type { ColorTokens } from '@/ir/schema';
+
+import { contrastRatio } from './contrast';
+
+export type ContrastWarning = {
+  /** Stable identifier so the UI can dedupe. */
+  id: string;
+  /** Short human-readable label, e.g. "Brand on surface". */
+  label: string;
+  ratio: number;
+  needs: number;
+  severity: 'fail' | 'low';
+};
+
+/**
+ * Lint a resolved color set for poor contrast on visible UI pairs. Auto-contrast
+ * already protects body text, so this focuses on brand/accent against surface
+ * (used for stats values, CTAs, kickers) where a low ratio still renders but
+ * looks washed out.
+ */
+export function lintColors(colors: ColorTokens): ContrastWarning[] {
+  const checks: Array<{ id: string; label: string; fg: string; bg: string; needs: number }> = [
+    {
+      id: 'brand-surface',
+      label: 'Brand on surface',
+      fg: colors.brand,
+      bg: colors.surface,
+      needs: 3.0,
+    },
+    {
+      id: 'accent-surface',
+      label: 'Accent on surface',
+      fg: colors.accent,
+      bg: colors.surface,
+      needs: 3.0,
+    },
+    {
+      id: 'brand-surface-muted',
+      label: 'Brand on surface-muted',
+      fg: colors.brand,
+      bg: colors.surfaceMuted,
+      needs: 3.0,
+    },
+    {
+      id: 'text-muted-surface',
+      label: 'Muted text on surface',
+      fg: colors.textMuted,
+      bg: colors.surface,
+      needs: 3.0,
+    },
+  ];
+
+  const warnings: ContrastWarning[] = [];
+  for (const c of checks) {
+    const ratio = contrastRatio(c.fg, c.bg);
+    if (ratio < c.needs) {
+      warnings.push({
+        id: c.id,
+        label: c.label,
+        ratio: Math.round(ratio * 10) / 10,
+        needs: c.needs,
+        severity: ratio < c.needs * 0.7 ? 'fail' : 'low',
+      });
+    }
+  }
+  return warnings;
+}

tests/render/lint.test.ts

@@ -0,0 +1,34 @@
+import { describe, expect, it } from 'vitest';
+
+import { lintColors } from '@/render/lint';
+
+const baseColors = {
+  brand: '#2563eb',
+  accent: '#7c3aed',
+  surface: '#ffffff',
+  surfaceMuted: '#fafafa',
+  text: '#0a0a0a',
+  textMuted: '#525252',
+  border: '#e5e5e5',
+  success: '#16a34a',
+  warn: '#d97706',
+  danger: '#dc2626',
+};
+
+describe('lintColors', () => {
+  it('returns no warnings for a balanced default palette', () => {
+    expect(lintColors(baseColors)).toEqual([]);
+  });
+
+  it('flags poor brand-on-surface contrast', () => {
+    const bad = { ...baseColors, brand: '#fafafa' };
+    const warnings = lintColors(bad);
+    expect(warnings.some((w) => w.id === 'brand-surface')).toBe(true);
+  });
+
+  it('flags poor muted-text contrast', () => {
+    const bad = { ...baseColors, textMuted: '#e5e5e5' };
+    const warnings = lintColors(bad);
+    expect(warnings.some((w) => w.id === 'text-muted-surface')).toBe(true);
+  });
+});