Understanding the DOM Tree and Rendering Tree
When you open a web page, have you ever wondered how the browser transforms raw HTML, CSS, and JavaScript into a fully rendered website? This process involves several steps, including the construction of the Document Object Model (DOM) and the rendering tree. In this article, we will explore how browsers render web pages, the role of the DOM and CSSOM, and factors that affect rendering performance.
What is the DOM?
The Document Object Model (DOM) is a W3C standard that provides a structured representation of an HTML or XML document. It allows programs and scripts to dynamically access, modify, and update the content, structure, and style of a document.
When a web page loads, the browser parses the HTML and constructs the DOM tree, representing the hierarchical structure of the document. Each HTML element becomes a node in this tree, allowing for dynamic interactions using JavaScript.
Example HTML Code and DOM Tree
<html>
<head>
<title>My document</title>
</head>
<body>
<h1>Header</h1>
<p>Paragraph</p>
</body>
</html>
The corresponding DOM tree:
How Does the Browser Render HTML?
A web browser downloads and processes files from a remote server, converting them into a displayable format. The browser engine performs this process by:
- Downloading and parsing HTML → Constructs the DOM tree.
- Downloading and parsing CSS → Constructs the CSS Object Model (CSSOM).
- Combining the DOM and CSSOM → Creates the Render Tree.
- Calculating layout → Determines the position of elements.
- Painting → Renders the final visual representation on the screen.
Stages of Creating the DOM Tree
- Bytes to Characters → The browser reads HTML as raw bytes and converts them into characters.
- Characters to Tokens → The characters are converted into tokens representing different HTML elements.
- Tokens to Nodes → These tokens are then transformed into nodes, forming the DOM tree.
- DOM Tree Construction → The hierarchical structure of the document is finalized.
What is the CSSOM?
The CSS Object Model (CSSOM) represents the styles associated with different HTML elements. Similar to the DOM, the CSSOM is constructed in multiple steps:
- Parsing CSS bytes → Converts raw CSS into characters.
- Tokenization → Breaks characters into tokens.
- Building nodes → Constructs the CSSOM tree.
The CSSOM allows browsers to determine the final appearance of the elements in a web page.
What is the Render Tree?
The Render Tree is a combination of the DOM and CSSOM. It contains all visible DOM content along with the computed styles needed for rendering. Elements with display: none; are omitted from the Render Tree, whereas elements with visibility: hidden; remain but do not appear visually.
Layout and Paint Steps
Once the Render Tree is built, the browser follows two additional steps to display the webpage:
- Layout (Reflow): Determines the exact size and position of each element.
- Painting: The browser paints pixels on the screen based on the Render Tree.
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