How CSS-in-JS Handles Server-Side Rendering

CSS-in-JS libraries like Styled-Components handle server-side rendering (SSR) by extracting critical styles during the initial HTML generation, injecting them into the document <head>, and synchronizing them during client-side hydration. This process prevents a Flash of Unstyled Content (FOUC) and ensures that the browser receives a fully styled HTML document before any client-side JavaScript executes.

The Challenge of CSS-in-JS on the Server

In a standard client-side Single Page Application (SPA), CSS-in-JS libraries evaluate styles dynamically in the browser. When a component mounts, the library parses its CSS rules, generates a unique hashed class name, and injects the rules directly into the DOM via <style> tags.

On the server, there is no active DOM tree. If a server renders React components to static markup using methods like renderToString, the JavaScript-driven styling mechanism does not automatically produce a stylesheet. Without explicit server-side handling, the browser renders raw, unstyled HTML until the JavaScript bundle downloads, parses, and injects the styles on the client.

The Extraction and Injection Workflow

Libraries like Styled-Components, Emotion, and others solve this problem using a dedicated server-side registry. In Styled-Components, this is handled through the ServerStyleSheet API. The process operates in five primary steps:

  1. Creating the Style Registry: Before rendering the component tree on the server, a new stylesheet collector instance is initialized. This instance acts as a temporary in-memory store for all CSS rules generated during the render pass.
  2. Collecting Styles During Render: The application root is wrapped with the stylesheet collector's provider component. When ReactDOMServer.renderToString() traverses the component tree, every styled component registers its generated CSS rules and unique class names with the in-memory stylesheet.
  3. Extracting the CSS: Once the HTML string generation completes, the stylesheet instance exposes methods—such as getStyleTags() or getStyleElement()—to retrieve all registered rules formatted as inline HTML <style> tags.
  4. HTML Document Assembly: The extracted style tags are placed directly into the <head> section of the server-rendered HTML template, while the component markup is placed inside the root container. The complete document is then sent to the client.
  5. Rehydration on the Client: When the client-side JavaScript loads, the CSS-in-JS runtime detects the existing server-rendered <style> tags. Instead of regenerating or duplicating these styles, the library rehydrates its internal style map, attaches event listeners, and resumes dynamic styling operations seamlessly.

Critical CSS by Default

Because styles are only registered when a component actually renders, the server-side extraction process automatically performs critical CSS extraction. Unused styles for components not present in the current view are excluded from the initial payload, keeping page weight low and improving First Contentful Paint (FCP) metrics.

Zero-Runtime Alternative Approaches

While runtime libraries like Styled-Components rely on server-side registry collectors, newer CSS-in-JS solutions (such as Vanilla Extract, StyleX, or Linaria) handle SSR at build time. These libraries compile styles into static .css files during the build step, eliminating the need for dynamic server-side extraction collectors and relying instead on standard <link rel="stylesheet"> tags.