Browser Rendering Process

-

Browser Rendering Process

The browser rendering process is how a browser takes the HTML, CSS, and JavaScript of a webpage and turns it into pixels that we see on the screen. This involves multiple steps, including parsing, rendering, and handling updates like repaints, reflows, and composition. Let’s go through this process step by step in simple terms.

Step 1: Parsing the HTML

  1. HTML Parsing:

    • The browser reads the HTML code line by line and converts it into a structure called the DOM Tree (Document Object Model).
    • Example:
      <div>
  <h1>Hello</h1>
  <p>World</p>
</div>
      The DOM Tree looks like:

      <html>
  <body>
    <div>
      <h1>Hello</h1>
      <p>World</p>
    </div>
  </body>
</html>

  2. CSS Parsing:

    • The browser reads the CSS and creates a CSSOM Tree (CSS Object Model), which represents the styles applied to each element.
    • Example:
      h1 {
  color: red;
}
p {
  font-size: 16px;
}
      CSSOM Tree:
      h1 -> color: red
p  -> font-size: 16px

Step 2: Constructing the Render Tree

  1. The browser combines the DOM Tree and CSSOM Tree to create the Render Tree.

    • The Render Tree determines which elements are visible and how they should be styled.
    • Hidden elements (e.g., display: none) are excluded from the Render Tree.

  2. Example: For this HTML:


    <div style="display: none;">
  <p>Hidden</p>
</div>
<h1 style="color: red;">Visible</h1>

    Render Tree:


    <h1> -> color: red

Step 3: Layout (Reflow)

  1. What is Layout?

    • The browser calculates the exact position and size of each element on the screen based on the Render Tree.
    • This process is called Layout or Reflow.

  2. Example: For this HTML:


    <div style="width: 100px;">
  <p style="width: 50px;">Text</p>
</div>
    • The browser determines:
      • The <div> is 100px wide.
      • The <p> is 50px wide and positioned inside the <div>.

  3. Layout involves:

    • Box dimensions (width, height, margin, padding, etc.).
    • Element positioning (top, left, right, bottom).

Step 4: Painting

  1. What is Painting?

    • After the layout is calculated, the browser fills in the pixels to render colors, text, borders, shadows, and images on the screen.
    • This step draws the visual representation of the elements.

  2. Example: For this CSS:


    h1 {
  color: red;
  background: yellow;
}

    The browser paints:

    • A yellow background.
    • Red text.

Step 5: Composition

  1. What is Composition?

    • Modern browsers render webpages in layers (like Photoshop layers).
    • Composition is the process of combining these layers to create the final image seen on the screen.
    • This step is handled by the GPU (Graphics Processing Unit) for better performance.

  2. Why Layers?

    • Some elements (like position: fixed, transform, or animations) are placed in separate layers to optimize rendering.

Browser Lifecycle: Repaints, Reflows, and Composition

When changes happen on the page (e.g., styles are updated, new elements are added, or animations run), the browser decides what needs to be updated. These updates can trigger repaints, reflows, or compositions.

1. Reflow (Layout)

  1. What is Reflow?

    • Reflow happens when the browser needs to recalculate the layout of the page because something changed.
    • Example:
      • Adding a new element.
      • Resizing the browser window.
      • Changing dimensions (like width or height).

  2. Expensive Operation:

    • Reflow involves recalculating positions and sizes of all affected elements, so it’s computationally expensive.

  3. Example of Reflow:


    const div = document.getElementById("myDiv");
div.style.width = "200px"; // Triggers reflow because layout changes

2. Repaint

  1. What is Repaint?

    • Repaint happens when visual properties (like color, background, or visibility) are updated, but the layout stays the same.
    • Example:
      • Changing color or background-color.
      • Updating box-shadow.

  2. Less Expensive:

    • Repaint only redraws the affected elements without recalculating the layout.

  3. Example of Repaint:


    const div = document.getElementById("myDiv");
div.style.backgroundColor = "red"; // Triggers repaint only

3. Composition

  1. What is Composition?

    • Composition happens when changes are made to elements in separate layers, and the browser re-combines the layers without repainting or reflowing.
    • Example:
      • Transformations (transform, translate, rotate, etc.).
      • Animations or opacity changes.

  2. Very Fast:

    • Composition is GPU-accelerated and doesn’t require recalculating layout or repainting.

  3. Example of Composition:


    const div = document.getElementById("myDiv");
div.style.transform = "translateX(50px)"; // Only composition occurs

Optimization Tips

  1. Avoid Triggering Reflows:

    • Use transform or opacity for animations instead of width or height.
    • Batch DOM updates to avoid multiple reflows.

  2. Minimize Repaints:

    • Use classes to update styles in bulk instead of modifying individual properties.

  3. Enable GPU Acceleration:

    • Use CSS properties like transform or will-change to move animations to a separate layer.

Summary

  1. Parsing: HTML and CSS are converted into DOM and CSSOM trees.
  2. Render Tree: DOM and CSSOM are combined to create the Render Tree.
  3. Layout (Reflow): Browser calculates the size and position of elements.
  4. Painting: Visual elements are drawn (colors, borders, text, etc.).
  5. Composition: Layers are combined into the final image.

By understanding these steps, you can write more efficient and performant code while avoiding costly browser updates like reflows and repaints. Let me know if you want code examples or more clarification!

Comments

Post a Comment

Popular posts from this blog

JavaScript Must-Read Topics for Senior Developer Interviews

-
JavaScript Must-Read Topics for Senior Developer Interviews Core JavaScript Variables and Scoping ( var , let , const ) ✅ Hoisting✅ Closures✅ The Event Loop✅ Promises✅ and Async/Await✅ Callbacks✅ Prototype and Prototypal Inheritance✅ this Keyword and Binding✅ Execution Context and Call Stack✅ Error Handling ( try-catch , finally , custom errors)✅ ES6+ Features (Destructuring✅, Spread/Rest✅, Arrow Functions✅, etc.) Type Coercion and Type Checking ✅ == vs ===  ✅ Object-Oriented Programming in JavaScript (Classes, Constructors)✅ Functional Programming Concepts (Higher-Order Functions✅, Pure Functions✅, Immutability✅) Module Systems ( require , import/export , CommonJS, ESModules)✅ JavaScript Design Patterns (Singleton, Factory, Observer, etc.) Garbage Collection✅ Advanced Topics Currying✅ Memoization✅ Throttling and Debouncing✅ Event Bubbling , Event Capturing and Event Delegation✅ Shadow DOM and Web Components Iterators, Iterables and Generator✅ WeakMap and WeakSet✅ Proxy and...
Read more

React.js Topics for a Mid-Senior Level Developer Interview Preparation

-
  React.js Topics for a Mid-Senior Level Developer Interview Preparation 1. Core Concepts React Component Types (Class vs Functional Components)✅ JSX and Rendering Elements✅ Props and Prop Validation✅ State Management (useState, class-based state)✅ Lifecycle Methods vs Hooks (useEffect)✅ 2. Advanced React Context API✅ Higher-Order Components (HOC)✅ Render Props✅ React Portals✅ Refs (createRef✅, useRef✅, forwardRef✅) Error Boundaries✅ Suspense and Lazy Loading✅ Strict Mode✅ Concurrent Rendering✅ 3. React Hooks Basic Hooks: useState, useEffect✅ Advanced Hooks: useContext✅, useReducer✅, useMemo✅, useCallback✅, useRef✅, useLayoutEffect✅ Custom Hooks (Creation and Use Cases)✅ 4. React Performance React.memo and useMemo✅ React Profiler Avoiding Re-renders (key, memoization, and immutability)✅ Code Splitting and Lazy Loading✅ Virtual DOM and Reconciliation✅ 5. Routing React Router (v6+): Route Configurations✅ Nested Routing✅ Dynamic Routing✅ Programmatic Navigation (useNavigate, useParams...
Read more

Must read Topics for Mid Level Node.js Developer Interview Preparation

-
  Must read Topics for Mid Level Node.js Developer Interview Preparation 1. Core Node.js Fundamentals Node.js Architecture (Single-threaded, Event Loop, Non-blocking I/O)✅ Understanding require and ES Modules ( import/export )✅ Core Modules: fs path os events http / https stream (Readable, Writable, Transform, Duplex) Process Management: process (environment variables, process.nextTick , process.exit ) Handling Uncaught Exceptions Timers ( setTimeout , setInterval , setImmediate ) Working with Buffers Working with the File System ( fs.promises , async/await with fs ) Child Processes ( child_process , exec , spawn , fork ) 2. Package Management with npm and Yarn Managing dependencies ( package.json , package-lock.json )✅ Semantic Versioning ( ^ , ~ , exact versions)✅ Local vs. global packages✅ Scripts in package.json (e.g., npm run dev )✅ Using npx ✅ 3. Asynchronous Programming Callbacks✅ Promises✅ async/await ✅ Event Emitters Streams and Pipelining Error handling in async code...
Read more