Definition
Rendering is the process by which the raw code of a web page (HTML, CSS and JavaScript) is turned into the actual content and layout a visitor or crawler gets to see. For SEO it comes down to one question: is the content already in the HTML the server sends, or is it only built in the browser by JavaScript?
With server-side rendering (SSR) or static generation (SSG), the server builds the complete HTML and sends it ready-made. Every crawler sees the text, links and metadata immediately. With client-side rendering (CSR), the server sends a nearly empty HTML shell with a JavaScript bundle; the browser executes it and only then builds the content. Modern frameworks such as Next.js, Nuxt and Astro combine both: the first view comes from the server, and JavaScript takes over interaction afterwards (hydration).
Google can execute JavaScript, but does so in a second step. First the HTML is crawled and indexed; the page then enters a render queue, where a headless Chrome runs the JavaScript and processes the resulting content. That second step can take hours to days and fails on timeouts, blocked scripts or errors. Content that only exists after JavaScript is therefore indexed later, less completely, or sometimes not at all.
For AI engines, the problem is bigger. GPTBot, ClaudeBot, PerplexityBot and most other AI crawlers do not execute JavaScript: they only read the HTML the server returns. A page that loads its text, prices or FAQ client-side is empty to them. This is one of the most common reasons pages rank fine in Google but are never cited in AI answers.
The test is simple: view the page source (not the DOM in DevTools) or fetch the URL with curl, and check whether your core text, headings, internal links and structured data are in it. If not, that content needs to move server-side. Traze publishes pages as complete HTML with all content, metadata and JSON-LD in the server response, and checks in the technical scan whether important content depends on JavaScript.