# Rendering Engine Basics The rendering engine, also known as the layout engine or browser engine, is a key component in modern web browsers. It transforms the HTML, CSS, and JavaScript of a webpage into visual and interactive content. ## Layout Process The engine starts by parsing HTML and CSS code, creating two structures: the Document Object Model (DOM) and the CSS Object Model (CSSOM). The DOM represents the HTML structure, while the CSSOM represents the styling information. These two structures are combined into a render tree. ```javascript //Sample DOM creation let dom = document.createElement('div'); dom.innerHTML = '<p>Test</p>'; document.body.appendChild(dom); ``` ## Render Tree The render tree is a visual representation of the webpage. It consists of visual CSS properties, such as display, color, and width. The engine will remove any elements with the 'display: none' property, as they do not contribute to the visual output. ```css /*Sample CSSOM creation*/ div { display: block; color: red; width: 100px; } p { display: none; } ``` ## Layout and Painting Layout, also known as reflow, is the process where the engine calculates the exact position and size of each object in the render tree. It starts from the root and works its way down. After layout comes painting. The engine paints each node according to the visual properties defined in the render tree. Complex features like gradients or shadows require more painting operations, affecting performance. ```javascript // Triggering Layout and Paint dom.style.width = '200px'; // Triggers layout and paint dom.style.opacity = '0.5'; // Triggers paint ``` ## Compositing Finally, the composited layers are drawn onto the screen. This process is highly optimized in modern browsers, often using GPU acceleration for better performance. Understanding the basics of the rendering engine allows you to create web content that is not only visually appealing, but also optimized for performance.