The critical rendering path
refers to the sequence of steps that a web browser takes to render and display a web page on the user's screen. Let's walk through an example to understand it better:
<link rel="stylesheet" href="styles.css">
<h1>Welcome to the Example Page</h1>
<p>This is some content on the page.</p>
The critical rendering path for this example would involve the following steps:
1. HTML Parsing:
The browser starts fetching the HTML file and parses it from top to bottom. It constructs the Document Object Model (DOM) tree, which represents the structure and content of the web page. In this case, it would create elements for `<html>`, `<head>`, `<title>`, `<link>`, `<body>`, `<h1>`, `<p>`, and `<script>`, among others.
2. CSS Parsing and Rendering:
While parsing the HTML, the browser encounters the CSS file specified in the `<link>` tag. It fetches and parses the CSS file to construct the CSS Object Model (CSSOM). The browser then combines the DOM and CSSOM to create the Render Tree, which represents the visual elements to be displayed on the page. It applies the styles from the CSS to the corresponding DOM elements.
3. Layout and Painting:
The browser performs layout calculations to determine the size and position of each element in the Render Tree. This process is called layout or reflow. Once the layout is complete, the browser proceeds to the painting phase, where it fills in pixels on the screen to render the visible content. In this step, the browser renders the text, headers, paragraphs, and other elements according to their calculated layout.
By optimizing the critical rendering path
, you can reduce the time it takes for a web page to be visually usable, improve perceived performance, and enhance the overall user experience.