Repaint vs Reflow: How Browser Rendering Affects Website Performance

When a browser loads a webpage, it builds the DOM and CSSOM, calculates element positions, and draws pixels on screen — a process involving two key operations: repaint and reflow. A repaint occurs when only visual properties like color or opacity change, requiring the browser to redraw pixels without recalculating layout, making it relatively fast. A reflow is triggered by changes to an element's size, position, or structure, forcing the browser to recalculate the entire layout before repainting, which is more computationally expensive. Developers can minimize performance costs by batching DOM reads and writes, using CSS classes instead of multiple inline style changes, and preferring CSS transforms over positional properties like 'left' or 'top' for animations. While modern JavaScript engines such as Chrome's V8 and Firefox's SpiderMonkey automatically batch some layout recalculations, forcing synchronous reflows by reading layout properties immediately after style changes can still degrade performance across all browsers.