Fix Cumulative Layout Shift (CLS) — Claude Code

Q: How do I set dimensions on responsive images?

Use `width` and `height` attributes with the raw dimensions, plus CSS `height: auto`. Browser calculates aspect ratio and reserves space.

Q: font-display: swap or optional?

`swap` is standard but can cause CLS. `optional` prevents shift but may not load the custom font on slow connections. Test both.

CLS happens when content jumps during load — you click something and hit the wrong element. Google penalizes it; users hate it.

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Fixing this in Claude Code

Anthropic's official terminal coding agent

Claude Code can grep the whole codebase first, plan the change, then edit every affected file. Paste the prompt below into your Claude Code chat and the fix rolls out across the project in one pass.

Using a different tool? Pick your stack:

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The prompt for Claude Code

Copy and paste this into your Claude Code chat exactly as-is.

Run Claude Code in the project root and make these exact changes, scanning the whole codebase first:

Fix Cumulative Layout Shift

1. Add explicit width and height to all images and video elements.
2. Avoid inserting content above existing content dynamically.
3. Use font-display: swap for web fonts.

Why this matters

CLS is the "janky load" metric. When images load without declared dimensions, the page height jumps — and users often click the wrong button mid-shift.

Core Web Vital — Google rewards CLS < 0.1 and penalizes > 0.25. Easy fix: explicit dimensions everywhere.

How to use this prompt in Claude Code

1. Open your Claude Code project.
2. Copy the prompt above with the copy button.
3. Paste into the Claude Code chat and send.
4. Review the diff, accept the changes, redeploy.
5. Verify the fix using the checklist below.

Common mistakes to avoid

error_outlineImages without width/height — the #1 cause of CLS.
error_outlineLate-loading web fonts that swap and shift text.
error_outlineInjecting banner/cookie notices above existing content.
error_outlineAdding ads or embeds without reserved space.

How to verify the fix worked

check_circlePageSpeed Insights → CLS < 0.1.
check_circleDevTools Performance → Experience → Layout Shift events — none over 0.05.
check_circleRecord a slow mobile load (4G throttle) — no visible jumps.

Frequently asked questions

How do I set dimensions on responsive images?expand_more

Use `width` and `height` attributes with the raw dimensions, plus CSS `height: auto`. Browser calculates aspect ratio and reserves space.

font-display: swap or optional?expand_more

`swap` is standard but can cause CLS. `optional` prevents shift but may not load the custom font on slow connections. Test both.

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