How does CSS actually work (under the hood of browser)?

CSS works by the browser’s rendering engine parsing and applying CSS rules to HTML elements to determine how they should be styled and displayed. Here’s a simplified overview of how CSS works under the hood in a browser:

  1. Parsing: When a web page is loaded, the browser’s rendering engine parses the HTML and builds a Document Object Model (DOM) tree, which represents the structure and content of the page. The rendering engine then parses the CSS stylesheets associated with the page.
  2. Selector Matching: The rendering engine applies CSS selectors to the elements in the DOM tree to determine which styles apply to each element. It matches the selectors to the corresponding elements based on the selector specificity rules.
  3. Style Computation: Once the selector matching is complete, the rendering engine computes the final styles for each element. It combines the styles from different sources, such as inline styles, external stylesheets, and styles inherited from parent elements. If there are conflicting styles, the cascade and specificity rules determine which style takes precedence.
  4. Layout (Reflow): After the styles are computed, the rendering engine performs a layout (also known as reflow) process to determine the position and size of each element on the page. It calculates the dimensions of the elements based on their styles, content, and other factors. The layout process takes into account the box model, positioning properties, floats, and other layout rules.
  5. Painting: Once the layout is determined, the rendering engine performs the painting phase. It traverses the DOM tree, applies the computed styles, and paints the pixels on the screen accordingly. This involves rendering text, applying background colors, borders, shadows, and other visual properties.
  6. Rendering Updates: As the user interacts with the page, the rendering engine may need to update and repaint portions of the page. This happens in response to user actions (e.g., scrolling, resizing the window) or changes to the page’s content or styles. The rendering engine optimizes the updates to minimize the performance impact.

It’s important to note that the actual rendering process involves more complexity and optimization techniques than outlined here. Browser engines, such as Blink (used by Chrome and Opera), Gecko (used by Firefox), and WebKit (used by Safari), implement various rendering optimizations to enhance performance and provide a smooth user experience.

Understanding how CSS works under the hood can help developers optimize their stylesheets, avoid rendering bottlenecks, and create efficient and visually appealing web pages.

error: Content is protected !!