How Do Web Browsers Work

Short answer

A web browser fetches resources from a server, parses HTML and CSS, executes JavaScript, and renders a visual page on your screen.

Steps

1. The browser resolves the domain via DNS and sends an HTTP request.
2. It receives HTML and begins parsing it into a DOM tree.
3. CSS is parsed into a CSSOM tree; the two are combined into a render tree.
4. JavaScript may modify the DOM and CSSOM during parsing.
5. The browser calculates layout, paints pixels, and composites layers.

Tips

• Modern browsers use multiple processes for tabs, improving stability.
• The Critical Rendering Path is the sequence that produces the first visible pixel.
• GPU acceleration handles compositing for smooth animations.

Common issues

• Render-blocking resources delay the first paint.
• Heavy JavaScript can cause frame drops and jank.