What does this page make a browser download?
Paste a URL to see how heavy the document is, how much CSS blocks first paint, and which images ship without dimensions, without srcset or without lazy loading — the markup mistakes that cause layout shift.
How this tool works, and what it cannot see
It measures the HTML transfer, fetches the linked stylesheets to size them, and audits every <img> tag in the markup.
It does not execute JavaScript and does not download images or scripts, so it reports structure, not a total page weight. Text sizes are uncompressed bytes.
For field data — real Core Web Vitals from real visitors — use PageSpeed Insights. This tool is for the fixable mistakes visible in markup.
FAQ
Why do missing width and height attributes matter?
Without them the browser cannot reserve space before the image loads, so everything below it jumps when it arrives. That jump is Cumulative Layout Shift, and it is one of the three Core Web Vitals.
Should every image be lazy-loaded?
No — lazy-loading anything in the first viewport delays your Largest Contentful Paint. Load the hero eagerly (ideally with fetchpriority="high") and lazy-load the rest.
Is this a Lighthouse replacement?
No. Lighthouse runs the page; this reads it. The two find different problems, and markup problems are the ones you can fix without a profiler.