Gate StandardSite subscribe button custom theme on AAA contrast (#10735)

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

src/alf/index.tsx

@@ -9,7 +9,12 @@ import {
   setFontScale as persistFontScale,
 } from '#/alf/fonts'
 import {themes} from '#/alf/themes'
-import {darken, lighten, rgbToHex} from '#/alf/util/colorGeneration'
+import {
+  contrastRatio,
+  darken,
+  lighten,
+  rgbToHex,
+} from '#/alf/util/colorGeneration'
 import {type Device} from '#/storage'
 
 export {type TextStyleProp, type Theme, type ViewStyleProp} from '@bsky.app/alf'
@@ -26,6 +31,7 @@ export const utils = {
   rgbToHex,
   lighten,
   darken,
+  contrastRatio,
 }
 
 export type Alf = {

src/alf/util/colorGeneration.test.ts

@@ -1,4 +1,10 @@
-import {darken, hexToRgb, lighten, rgbToHex} from './colorGeneration'
+import {
+  contrastRatio,
+  darken,
+  hexToRgb,
+  lighten,
+  rgbToHex,
+} from './colorGeneration'
 
 describe('hexToRgb', () => {
   it('parses 6-digit hex', () => {
@@ -92,3 +98,33 @@ describe('lighten / darken', () => {
     expect(darken('#zzz', 10)).toBe('#zzz')
   })
 })
+
+describe('contrastRatio', () => {
+  it('returns 21 for black on white', () => {
+    expect(contrastRatio('#000000', '#ffffff')).toBeCloseTo(21, 5)
+  })
+
+  it('returns 1 for identical colors', () => {
+    expect(contrastRatio('#abcdef', '#abcdef')).toBeCloseTo(1, 5)
+  })
+
+  it('is symmetric regardless of argument order', () => {
+    expect(contrastRatio('#123456', '#fedcba')).toBeCloseTo(
+      contrastRatio('#fedcba', '#123456')!,
+      5,
+    )
+  })
+
+  it('clears AAA large text (4.5:1) for a high-contrast pairing', () => {
+    expect(contrastRatio('#1d3a5f', '#ffffff')!).toBeGreaterThanOrEqual(4.5)
+  })
+
+  it('fails AAA large text (4.5:1) for a low-contrast pairing', () => {
+    expect(contrastRatio('#777777', '#888888')!).toBeLessThan(4.5)
+  })
+
+  it('returns null for invalid hex input', () => {
+    expect(contrastRatio('not-a-color', '#ffffff')).toBeNull()
+    expect(contrastRatio('#ffffff', '#zzz')).toBeNull()
+  })
+})

src/alf/util/colorGeneration.ts

@@ -72,6 +72,48 @@ export function rgbToHex(r: number, g: number, b: number): string {
     .slice(1)}`
 }
 
+/**
+ * Computes the WCAG contrast ratio between two colors, ranging from 1 (no
+ * contrast) to 21 (maximum contrast, i.e. black on white). Returns null if
+ * either argument is not a valid hex color.
+ *
+ * @see https://www.w3.org/TR/WCAG21/#dfn-contrast-ratio
+ */
+export function contrastRatio(hexA: string, hexB: string): number | null {
+  const rgbA = hexToRgb(hexA)
+  const rgbB = hexToRgb(hexB)
+  if (!rgbA || !rgbB) return null
+  const luminanceA = relativeLuminance(rgbA)
+  const luminanceB = relativeLuminance(rgbB)
+  const lighter = Math.max(luminanceA, luminanceB)
+  const darker = Math.min(luminanceA, luminanceB)
+  return (lighter + 0.05) / (darker + 0.05)
+}
+
+/**
+ * Computes the WCAG relative luminance of an RGB color, ranging from 0 (black)
+ * to 1 (white).
+ *
+ * @see https://www.w3.org/TR/WCAG21/#dfn-relative-luminance
+ */
+function relativeLuminance({
+  r,
+  g,
+  b,
+}: {
+  r: number
+  g: number
+  b: number
+}): number {
+  const toLinear = (channel: number) => {
+    const normalized = channel / 255
+    return normalized <= 0.03928
+      ? normalized / 12.92
+      : ((normalized + 0.055) / 1.055) ** 2.4
+  }
+  return 0.2126 * toLinear(r) + 0.7152 * toLinear(g) + 0.0722 * toLinear(b)
+}
+
 function rgbToHsl(
   r: number,
   g: number,