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What a browser does

When you enter a URL, the browser coordinates network, security, document analysis and rendering.

From URL to response​

  1. It analyzes the URL: protocol, domain, port, path and parameters.
  2. It searches for the IP address of the domain using DNS.
  3. It establishes a secure connection for HTTPS.
  4. It sends an HTTP request.
  5. It receives a status code, headers and content.

Connections can be reused. With HTTP/2 or HTTP/3, several exchanges can share the same connection; so she is not systematically closed after each resource.

From response to pixels​

  • the HTML becomes a DOM;
  • the CSS becomes a CSSOM;
  • the browser combines the information necessary for rendering;
  • it calculates the geometry of the elements (layout);
  • he paints then composes the different layers;
  • JavaScript can modify the document and trigger new calculations.
HTML → DOM ─┐
            ├→ arbre de rendu → layout → paint → composition
CSS  → CSSOM┘

The role of JavaScript​

JavaScript can listen to interactions, modify the DOM, read some APIs of the browser and make new requests. A long JavaScript task may, however, temporarily block display and interactions.

Quick challenge​

Open developer tools, reload a page and choose its query Main HTML. Find its HTTP code, content type and duration, then identify a resource then loaded by the browser.