Use when applies to all CSS `font-size` declarations, particularly those using `px` units for body text, UI labels, and navigation. Use when auditing mobile responsiveness and accessibility compliance. Check for `font-size` in media queries targeting small screens.
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Approximately 1 in 3 adults over 65 increases their browser's default font size. Low-vision users who cannot use screen magnifiers rely on browser font size settings as their primary accessibility tool. If font sizes are set in px only, browser font size preferences are ignored entirely. Text smaller than 16px on mobile forces all users to zoom in, breaking the page layout and triggering horizontal scroll. iOS Safari's auto-inflation of sub-12px text causes unintended layout shifts.
font-size values below 12px; iOS Safari ignores values below 12px and auto-inflates themrem or em units instead of px so text scales with the user's browser font size preferencefont-size: 100% on <html> using an override that blocks user scaling — respect the browser default (16px)Find all CSS font-size declarations. Flag: (1) body text or paragraph text set below 16px (or 1rem); (2) any font-size below 12px; (3) font-sizes set in px on <html> or <body> that override the browser default; (4) font-sizes that do not scale proportionally when the browser zoom is set to 200% (test in Chrome DevTools by setting font-size in browser settings or using zoom). Check that text remains readable and layout does not break at 200% zoom.
(1) Convert pixel font sizes to rem units: divide the pixel value by 16 (the browser default) — e.g., 14px becomes 0.875rem. (2) Set body text to at least 1rem (16px equivalent) on mobile. (3) Replace html { font-size: 10px } (a common reset trick) with html { font-size: 62.5% } or avoid font-size resets entirely — instead use rem values calculated from the actual 16px base. (4) If small text is intentional (captions, footnotes), ensure it is at least 0.75rem (12px) and use font-size: 0.75rem so it scales with user preferences.
WCAG 2.1 SC 1.4.4 (Resize Text) requires that text can be resized up to 200% without loss of content or functionality, except for captions and images of text. Using px units for font sizes makes text immune to browser font size settings (though not to browser zoom). Using rem or em units allows text to scale with the root font size, which the user can adjust in browser settings — this is the preferred approach. iOS Safari also has a built-in heuristic that inflates text smaller than 12px when the page is displayed in mobile view, which can cause layout inconsistencies.
Review stylesheets, component styles, and responsive states related to Use readable font sizes on mobile. Flag exact selectors, declarations, or breakpoints that violate the rule in the rendered UI.
For full implementation details, code examples, and framework-specific guidance,
see references/rule.md.
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概要と使いどころ
Use when reviewing templates, rendered HTML, or shared components related to Create a custom 404 error page. Validate the final browser-facing markup, not just the source framework abstraction.
日本語の概要は準備中です。原文の説明を表示しています。
Use when auditing metadata, crawlability, structured data, or indexability related to Create a dedicated About page. Verify the rendered HTML and HTTP response rather than relying only on source files.
日本語の概要は準備中です。原文の説明を表示しています。
Use when reviewing component libraries, page flows, or CI pipelines that need repeatable accessibility checks. Automated testing is strongest at catching structural and attribute-level issues; it does not replace keyboard, screen reader, and manual UX testing.
日本語の概要は準備中です。原文の説明を表示しています。
Use when reviewing sign-in, sign-up, MFA, CAPTCHA, recovery, and re-auth flows. Evaluate the full authentication path, including error handling and backup methods, not just the primary login form.
日本語の概要は準備中です。原文の説明を表示しています。
Use when reviewing templates, rendered HTML, or shared components related to Make notifications accessible. Validate the final browser-facing markup, not just the source framework abstraction.
日本語の概要は準備中です。原文の説明を表示しています。
Use when reviewing rendered HTML, interactive components, or design-system patterns related to Use semantic table markup for screen readers. Check native semantics first, then inspect keyboard behavior, focus flow, accessible names, and screen-reader output where relevant.
日本語の概要は準備中です。原文の説明を表示しています。