Font Resizer Guide: How to Adjust Text Size for AccessibilityAccessibility isn’t an optional extra — it’s essential. For many people with low vision, dyslexia, age-related sight changes, or simply varying reading preferences, the ability to change text size reliably on websites and apps can mean the difference between understanding content and being excluded by it. This guide explains why font resizing matters, how it ties into accessibility standards, practical techniques for implementing and testing font resizers, and best practices to ensure text remains readable, usable, and attractive at any size.
Why font resizing matters
- Better readability for diverse users. Users with visual impairments or cognitive differences often need larger text to read comfortably. Allowing adjustable text size helps them access content without relying on browser zoom alone.
- Device and context flexibility. A user reading in bright sunlight on a phone may need larger text than someone reading on a desktop at home.
- Legal and ethical obligations. Accessibility laws and guidelines (such as WCAG) increasingly influence product requirements. Making text adjustable supports compliance and reduces legal risk.
- Improves engagement and retention. When text is readable, users spend more time on a site, complete tasks faster, and have fewer errors.
Accessibility standards and font resizing
- WCAG (Web Content Accessibility Guidelines) focuses on perceivable content. Key relevant points:
- Text should be resizable up to 200% without loss of content or functionality.
- Use relative units and avoid absolute positioning that breaks when text sizes change.
- Following these guidelines helps meet legal standards in many jurisdictions and ensures broader usability.
Technical approaches to font resizing
Below are common methods to implement text-resizing features, with pros and cons for each approach.
| Method | How it works | Pros | Cons |
|---|---|---|---|
| Browser zoom | Users use built-in browser zoom or OS-level scaling | Works everywhere without extra code | Can break fixed layouts; affects entire page, including images |
| CSS relative units (em, rem, %) | Define base sizes in rem/em and adjust root font-size | Predictable scaling; easy to control globally | Requires careful layout to avoid overflow |
| CSS variables | Use –base-size and compute fonts with calc() | Central control; dynamic updates via JS | Browser support is good but needs build-time thought |
| JavaScript UI controls | Add buttons/sliders that change root font-size or add classes | Direct UX control; can persist preferences | More code and testing; must handle edge cases |
| Accessibility settings integration | Respect user/OS preferences (prefers-reduced-motion, font-size settings) | Respects user choices at system level | Not all platforms expose granular font-size prefs |
Implementation patterns (with examples)
- Using rem and changing root font-size (recommended)
- Strategy: Define typography using rem units, set the html font-size to 100% (typically 16px), and modify html { font-size } dynamically.
- Why: rem is anchored to the root and keeps components proportional.
Example CSS:
html { font-size: 100%; } /* 16px by default */ body { font-family: system-ui, -apple-system, "Segoe UI", Roboto, "Helvetica Neue", Arial; line-height: 1.4; } h1 { font-size: 2rem; } /* 32px */ p { font-size: 1rem; } /* 16px */ Example JS to increase/decrease:
const root = document.documentElement; function setFontSizePercent(percent) { root.style.fontSize = `${percent}%`; } // increase 10% function increase() { const current = parseFloat(getComputedStyle(root).fontSize) / 16 * 100; setFontSizePercent(Math.min(200, current + 10)); } // decrease 10% function decrease() { const current = parseFloat(getComputedStyle(root).fontSize) / 16 * 100; setFontSizePercent(Math.max(50, current - 10)); } - Using CSS classes (theme-like)
- Strategy: Toggle classes like .font-large or .font-xlarge on the root and set font-size values in CSS.
- Why: Simple to implement and test; good for fixed options (small/medium/large).
Example:
html.font-large { font-size: 112%; } html.font-xlarge { font-size: 125%; } - Slider control with CSS variables
- Strategy: Expose a CSS variable (–base-size) and let a range input update it with JS for fine-grained control.
- Why: Smooth control and consistent scaling via variables.
Example:
:root { --base-size: 1rem; } html { font-size: calc(var(--base-size) * 16); } p { font-size: calc(1 * var(--base-size)); } JS:
const slider = document.querySelector('#font-slider'); slider.addEventListener('input', (e) => { document.documentElement.style.setProperty('--base-size', `${e.target.value}rem`); }); Design considerations
- Maintain readable line-lengths. If text scales large, lines can become too long; use max-widths for paragraphs (50–75 characters optimal).
- Preserve line-height and spacing. Use relative line-height (e.g., 1.4–1.6) to avoid cramped text when scaled.
- Avoid fixed-height containers and absolute positioning that clip text when enlarged.
- Test responsive breakpoints with increased font sizes — content flow should remain intact.
- Use clear controls and labels (e.g., “A−” and “A+” with visible focus states) and ensure keyboard and screen reader operability.
- Persist user preference (localStorage or server-side profile) so the setting persists across visits.
UX patterns for font resizer controls
- Place controls where they’re easily discoverable (header, accessibility menu, or settings).
- Offer at least three levels (small, default, large) and consider a slider for advanced control.
- Provide visual feedback (preview text) and a reset-to-default option.
- Respect reduced-motion and other OS accessibility settings; don’t override them.
Testing and validation
- Manual tests:
- Increase text to 200% and check all pages for clipped content, overlapping elements, and broken interactions.
- Test keyboard navigation and focus states after resizing.
- Test on multiple devices, screen sizes, and browsers.
- Automated checks:
- Use Axe, Lighthouse, or WAVE to flag issues related to reflow or accessibility.
- Write visual regression tests with resized root font-size to catch layout regressions.
- Real-world validation:
- Conduct usability testing with users who rely on larger text sizes.
Common pitfalls and how to avoid them
- Using px for most typography: switch to rem/em to enable predictable scaling.
- Fixed-width layouts and hard-coded heights: prefer fluid layouts and min/max values.
- Only scaling text visually without ensuring interactive elements (buttons, controls) remain touch-accessible — increase target sizes alongside text.
- Not testing different languages — some languages expand text length; ensure UI accommodates longer words and translations.
Example checklist before release
- Text scales to at least 200% without loss of content or functionality.
- All interactive elements remain reachable by keyboard and not obscured by larger text.
- Line-height and spacing adapt correctly at larger sizes.
- Controls to adjust font size are visible, keyboard accessible, and screen-reader labeled.
- Preferences persist across sessions if applicable.
- Automated and manual accessibility tests run and pass.
Conclusion
A robust font resizer improves accessibility, user satisfaction, and compliance with accessibility standards. The most maintainable approach combines CSS relative units (rem/em), a clear UI control (buttons or slider), and thorough testing across sizes and devices. Prioritize proportional scaling, avoid fixed dimensions, and ensure controls are discoverable and keyboard-accessible. With those practices in place, your site or app will be far more inclusive and usable for everyone.
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