Core Web Vitals

What this check looks at

Core Web Vitals is one of the Technical SEO checks. Whether engines can crawl and index the site at all. It is judged from what your site served at the time of the crawl, and a page we could not reach is reported as unreachable rather than skipped.

Why this one needs a real browser

Reading the HTML is not enough for this check. The answer only appears once a browser has executed the page, so we load it in headless Chromium, let the scripts run, and measure what the finished document actually contains.

This is the gap most audit tools miss. A page can serve an almost empty shell to a plain fetch and fill itself in afterwards, which looks fine to a human and looks like nothing to a crawler that does not execute scripts. We compare the two views precisely because the difference between them is where citations are lost.

What you get from it

• The free single URL tools infer this from the served HTML alone. A full audit renders the page in a real browser and compares the two, because rendering costs real time
• Compares the raw response against the rendered document, so a shell page cannot pass unnoticed
• Reports what a non-executing crawler would have seen, which is what several answer engines are

What it reports when it fails

These are the findings this check can raise, taken from the code that raises them rather than rewritten for the website. An ellipsis stands where the report fills in what it measured on your page.

• worth fixinglargest contentful paint …ms, Google's poor threshold is …ms
• notelargest contentful paint …ms, above the …ms good bar
• worth fixingcumulative layout shift …, content moves while it loads
• notecumulative layout shift …
• worth fixingtime to first byte …ms, the server is slow before rendering starts
• notethese are lab measurements from one render on this machine, the field data
• noteINP is the third Core Web Vital and is not measured here, it needs a real

Where it sits in the audit

See every technical check or the full catalogue of 77 checks. The documentation covers the order checks run in and how a finding is fingerprinted, and the about page explains why we publish the checks that do not run.