Why the raw HTML is different from what you see in a browser
A browser renders a page for you - running JavaScript, applying styles, building out dynamic content - but the raw HTML a server actually sends over the wire is what this tool shows. That raw response is exactly what search engine crawlers, scrapers, and many automated tools see first, before any client-side script has a chance to run.
What this is useful for
Confirming meta tags, canonical URLs, Open Graph tags, or structured data (schema.org JSON-LD) are actually present in the server response - not just rendered into the page later by JavaScript - is a common SEO debugging need this tool answers directly, without digging through browser developer tools or a "view-source:" prefix. It's equally useful for checking whether a page is properly server-rendering its core content, which matters for both SEO and sites that need to work with JavaScript disabled or limited.
Why some pages look "empty" or different from what you expect
Many modern sites render most of their visible content with client-side JavaScript frameworks, meaning the raw HTML this tool fetches can look sparse - a shell with little visible text - even though the page looks completely normal and full of content in a regular browser. That gap itself is useful information: it tells you how much of a page's content depends on JavaScript actually executing, which matters for how well search engines and non-JavaScript tools can read it.
The size limit
Pages are fetched up to roughly 1 MB of HTML; anything beyond that is truncated, with a note shown so you know the output was cut off rather than being the complete page.