Zoom Reflow

Support content reflow at 400% zoom

Users with low vision zoom to 400% to read content—horizontal scrolling at that level makes it impossible to track lines of text and navigate effectively.

Quick Reference

• Content must reflow to single column at 400% zoom (1280px width at 320px equivalent)
• No horizontal scrolling required to read content
• All functionality must remain accessible at high zoom levels
• Use relative units and fluid layouts that adapt to viewport

Check

Zoom the browser to 400% and verify content reflows to a single column without horizontal scrolling, truncation, or overlapping elements. Test all interactive features remain functional at this zoom level.

Fix

Use responsive CSS with relative units (rem, em, %) instead of fixed pixels. Implement fluid layouts that adapt to viewport width. Test with CSS media queries to ensure layouts reflow gracefully.

Explain

Explain how users with low vision rely on browser zoom up to 400% to read content, and why horizontal scrolling creates significant barriers for users who can only see a small portion of the screen at once.

Code Review

Review the rendered markup and interactive states that affect Support content reflow at 400% zoom. Flag exact elements, roles, labels, focus behavior, or keyboard interactions that violate the rule, and note how to verify the fix with browser accessibility tooling or assistive tech.

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For full implementation details, code examples, and framework-specific guidance, see references/rule.md.

Rule page: https://frontendchecklist.io/en/rules/accessibility/zoom-reflow

Support content reflow at 400% zoom

Content reflows when zoomed to 400% without requiring horizontal scrolling or loss of functionality.

Priority: critical · Difficulty: intermediate · Time: 30 min

--- At 400% zoom, content must reflow to fit the viewport width without horizontal scrolling.

Code Example

/* ❌ Bad: Fixed width layout */
.container {
  width: 1200px;
}

/* ✅ Good: Fluid layout with max-width */
.container {
  width: 100%;
  max-width: 1200px;
  padding: 0 1rem;
}

/* ❌ Bad: Fixed columns */
.grid {
  display: grid;
  grid-template-columns: 300px 600px 300px;
}

/* ✅ Good: Responsive columns */
.grid {
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(280px, 1fr));
  gap: 1rem;
}

Why It Matters

Users with low vision zoom to 400% to read content—horizontal scrolling at that level makes it impossible to track lines of text and navigate effectively.

WCAG Requirement

400% zoom on a 1280px wide viewport = 320px equivalent width. Content must be usable at this width.

Single Column Reflow

/* Reflow to single column on narrow viewports */
.two-column {
  display: flex;
  flex-wrap: wrap;
}

.sidebar {
  flex: 1 1 250px;
}

.main-content {
  flex: 2 1 300px;
}

/* At 400% zoom, both become full width */

Tables That Reflow

/* Responsive table approach */
@media (max-width: 600px) {
  table, thead, tbody, th, td, tr {
    display: block;
  }

  thead {
    position: absolute;
    left: -9999px;
  }

  tr {
    margin-bottom: 1rem;
    border: 1px solid #ccc;
  }

  td {
    position: relative;
    padding-left: 50%;
  }

  td::before {
    content: attr(data-label);
    position: absolute;
    left: 0.5rem;
    font-weight: bold;
  }
}

<table>
  <thead>
    <tr>
      <th>Name</th>
      <th>Price</th>
    </tr>
  </thead>
  <tbody>
    <tr>
      <td data-label="Name">Product A</td>
      <td data-label="Price">$29.99</td>
    </tr>
  </tbody>
</table>

Navigation Reflow

/* Horizontal nav becomes vertical at narrow widths */
.nav {
  display: flex;
  flex-wrap: wrap;
  gap: 0.5rem;
}

@media (max-width: 400px) {
  .nav {
    flex-direction: column;
  }
}

Images That Scale

/* Images scale with container */
img {
  max-width: 100%;
  height: auto;
}

/* Prevent overflow */
.content img {
  max-width: 100%;
  object-fit: contain;
}

Form Reflow

/* ❌ Bad: Fixed width inputs */
input {
  width: 400px;
}

/* ✅ Good: Flexible inputs */
input {
  width: 100%;
  max-width: 400px;
}

/* Stack label and input at narrow widths */
.form-group {
  display: flex;
  flex-wrap: wrap;
  gap: 0.5rem;
}

.form-group label {
  flex: 0 0 150px;
}

.form-group input {
  flex: 1 1 200px;
}

Testing at 400%

1. Set browser to 1280px width 2. Zoom to 400% (Ctrl/Cmd + Plus) 3. Content should display like 320px mobile view 4. Verify:

• No horizontal scrollbar
• All content readable
• No overlapping elements
• All interactions work

React Responsive Component

function ResponsiveLayout({ sidebar, main }: { sidebar: React.ReactNode; main: React.ReactNode }) {
  return (
    <div className="flex flex-wrap">
      <aside className="w-full md:w-64 flex-shrink-0">
        {sidebar}
      </aside>
      <main className="w-full md:flex-1 min-w-0">
        {main}
      </main>
    </div>
  )
}

Some content legitimately requires two-dimensional scrolling (maps, data tables, diagrams). These are exceptions to the reflow requirement, but provide alternatives where possible.

Exceptions

• Temporary or intentionally inert UI can be removed from the focus order, but only when the same state is also communicated clearly to assistive technology users.
• A focus-management issue should be evaluated in the rendered interaction, not only from static markup, because route changes, overlays, and JS timing can change the real behavior.
• If a component is both unlabeled and focus-broken, fix the stronger user-facing orientation problem first rather than reporting multiple secondary symptoms.

Verification

Automated Checks

• Inspect the browser accessibility tree or accessibility pane for the relevant element, role, or accessible name.
• Run an automated accessibility checker such as axe or Lighthouse where applicable.

Manual Checks

• Test the affected UI with keyboard-only navigation and confirm the rule holds in the rendered experience.
• Re-test one representative user flow with a screen reader if this rule affects a key interaction.