Web browsers perform a fascinating and complex series of steps to transform raw HTML code into the visually rich and interactive web pages we use every day. Understanding browser rendering and HTML parsing offers valuable insight for web developers aiming to optimize performance and user experience. Here's a deep dive into the core stages involved in how browsers handle and render HTML content.
1. Receiving and Parsing HTML to Build the DOM
The rendering process starts when a browser requests an HTML document from a web server. As HTML bytes stream in, the browser begins parsing them immediately — this parsing converts markup into an internal data structure called the Document Object Model (DOM). The DOM is a hierarchical tree representation of every HTML element on the page, capturing their relationships and nesting.
HTML parsing itself has two key parts:
- Tokenization: The raw HTML string is broken down into tokens such as start tags, end tags, attribute names, and attribute values.
- Tree construction: These tokens are processed sequentially to build the DOM tree, where each element becomes a node linked as parent or child to others.
The parser operates incrementally as data arrives to allow progressive rendering, meaning users may see content before the entire page finishes loading. However, scripts without async or defer attributes block HTML parsing until they are downloaded and executed, which can delay rendering.
2. Parsing CSS to Create the CSSOM
At the same time or shortly after parsing HTML, the browser fetches CSS files and parses them into another tree structure called the CSS Object Model (CSSOM). This model organizes all styles that apply to elements, including rules from external stylesheets, inline styles, and style blocks.
The CSSOM combined with the DOM forms the foundation for rendering the page with proper style.
3. Construction of the Render Tree
Next, the browser combines the DOM and CSSOM trees to build the render tree. Unlike the DOM, the render tree reflects only elements that will actually be visible on the page, excluding those styled with display: none or non-visual elements like <head>.
The render tree nodes contain both the content and the computed styles, mapping exactly what will appear on the screen.
4. Layout (Reflow)
With the render tree ready, the browser calculates the exact coordinates and sizes for every visible element. This layout stage determines the position of elements based on CSS rules like width, height, margin, padding, and more.
5. Painting
Armed with position and style data, the browser paints pixels for each element on the screen. This painting phase fills in colors, text, images, borders—every visual detail. Painting happens in layers to optimize redrawing, especially for complex or dynamic content.
6. Compositing and Display
Finally, the painted layers are composited together and sent to the GPU for rendering onto the device display, producing the final webpage view users interact with.
Important Notes and Considerations
- Progressive rendering: Browsers strive to show users something as soon as possible, often rendering partial content as resources load.
- Blocking resources: Scripts without async or defer block HTML parsing and thus delay rendering.
- Efficiency: The size and complexity of the DOM significantly impact parsing and rendering speed; optimizing HTML structure improves performance.
- Re-rendering: User interactions or JavaScript can alter the DOM or CSSOM, triggering partial or full re-layout and repaint cycles.
Final Thoughts
Browser rendering and HTML parsing involve a well-orchestrated sequence: streaming and parsing HTML into the DOM, parsing CSS into the CSSOM, merging into the render tree, calculating layout, painting pixels, and finally compositing for display. Mastery of these steps empowers developers to build faster, smoother web experiences.
This deep understanding also helps tackle performance bottlenecks like render-blocking scripts and heavy DOM trees, ensuring users get visually complete pages as quickly as possible.
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Top comments (2)
When your browser gets that first chunk of HTML from the server, it doesn’t just immediately start showing stuff on screen. It starts by parsing the HTML to build what’s called the DOM - basically the structure of the page. At the same time, it’s grabbing and decoding your CSS to build the CSSOM. Once both are ready, the browser merges 'em into a render tree - kinda like a blueprint of what needs to show up and how it should look. Next comes the layout phase, where it figures out where every element should go on the screen. Now, while the DOM and CSSOM can be built side by side, there’s a catch: if your page includes render-blocking scripts or heavy stylesheets, all that prep work gets delayed. That’s bad news for performance. If you're running a Single Page App you get zippy interactions after load - but watch out for that dreaded blank screen during the wait. Gotta hydrate it properly or do some server-side rendering magic to keep things snappy.
Thanks @onlineproxy for explaining in detail behind the scene. Thanks again for learning from community member!!!!