Core Web Vitals (CLS) Layout Shift Root Cause Predictor
Identify common Cumulative Layout Shift risks in HTML and CSS before they affect real users.
Predictor
Results
Findings
Introduction
Cumulative Layout Shift measures unexpected movement of page content. Layout shifts commonly happen when media loads without reserved space, fonts swap late, or scripts inject content above existing elements.
This tool helps you catch common static risk factors before deployment.
How it works
The tool parses your HTML and CSS locally. It then looks for patterns that frequently contribute to CLS: unsized media, unstable font configurations, and transitions that animate layout-affecting properties.
- HTML is parsed in an inert document.
- CSS is analyzed with browser CSS parsing where available.
- Risk scores are heuristic and intended for prioritization.
- No network requests are made.
How to use
- Paste page HTML.
- Add related CSS, including font-face rules if relevant.
- Run the prediction.
- Review findings and mitigation suggestions.
- Validate with Chrome DevTools or field data after fixing issues.
Practical example
An image without width and height attributes can push content downward when it loads. Adding explicit dimensions or an aspect ratio lets the browser reserve the correct space before the image arrives.
Use cases
- Reviewing templates before launch.
- Debugging CLS regressions in component markup.
- Auditing font-loading strategy.
- Checking embed containers for reserved space.
- Prioritizing performance fixes.
Best practices
- Add width and height attributes to media elements.
- Use aspect-ratio for responsive media.
- Reserve space for ads, embeds, banners, and dynamic widgets.
- Use font metric overrides to reduce font-swap shift.
- Prefer transform and opacity animations over layout animations.
Common mistakes
- Loading images without dimensions.
- Injecting banners or notifications above content.
- Using web fonts without fallback metric alignment.
- Animating height or margin unnecessarily.
- Embedding third-party widgets without placeholder space.
Limitations
- The tool does not measure actual CLS.
- JavaScript-driven layout changes are not fully analyzed.
- Third-party runtime behavior may still cause shifts.
- Heuristic scores should be confirmed with lab and field data.
Browser compatibility
The tool works in current versions of major browsers. The CLS recommendations are based on widely accepted Core Web Vitals guidance.
FAQ
Is my markup uploaded?
No. HTML and CSS are analyzed locally in your browser.
Does the score equal actual CLS?
No. The score is a heuristic risk estimate. Real CLS depends on rendering, network timing, device behavior, and user interactions.
Can it detect third-party widget shifts?
It can detect missing dimensions on embed containers, but it cannot predict every runtime behavior injected by third-party scripts.
Should I still use PageSpeed Insights?
Yes. Use this tool for early diagnosis and prioritization, then verify with DevTools, lab tests, and field Core Web Vitals data.