Traditional web pages deliver their content in the HTML document returned by the server. Modern JavaScript-heavy sites (React, Vue, Angular, Next.js with client-side rendering) often deliver an almost empty HTML shell and populate the page with content rendered by JavaScript in the browser. If a crawler does not execute JavaScript, it sees only the empty shell — missing all the text, links, and images that matter for indexing.
Googlebot does process JavaScript, but with a significant delay — pages are first crawled, then queued for rendering, which can lag by days or weeks. Bing and other crawlers process JavaScript less reliably.
What SearchVitals checks: The Architecture audit simulates a crawler request without JavaScript and compares the visible content to a full browser render. A material difference in word count or link count triggers a critical finding.
Solutions: Server-side rendering (SSR), static site generation (SSG), or dynamic rendering can ensure crawlers receive fully rendered HTML on the first request.