How Webpage Rendering Works and Why It Matters
Webpage rendering is a fundamental technique that defines how people interact with a website. Every stage, from code management to content presentation, has a direct impact on loading speed, layout correctness, and visual quality. Understanding how browsers analyze web pages aids in the identification of possible performance concerns as well as chances for efficiency improvements. This procedure is optimized to ensure that websites load quickly, run smoothly, and produce consistent results across several devices and browsers.
What Is Webpage Rendering?
Webpage rendering is the process of converting HTML, CSS, and JavaScript code into a page that users can view and interact with within their browser. If any step of this process is delayed or not completed correctly, problems like the Flash of Unstyled Content (FOUC) might occur, causing sections of the page to load without styles and impacting how the website looks.
Explanation of DOM, CSSOM, and Render Tree
Before we get into how webpage rendering works and how to optimize it, it’s important to grasp three key terms: DOM, CSSOM, and Render Tree. These are critical components of how a browser interprets and displays a webpage.
DOM (Document Object Model)
The DOM is a programming interface that represents the structure and content of a web page, typically written in HTML. It breaks the page down into individual nodes and objects, allowing programming languages like JavaScript to access, read, and modify different parts of the page. This structure makes it possible to interact with elements, update content, and change how the page looks or behaves without reloading it.
Understanding how the document object model works is essential for manipulating web elements effectively. Learn more about its role and how it impacts web development in this detailed article.
CSSOM (CSS Object Model)
The CSSOM is a set of application programming interfaces that handle the style information of a webpage. It contains all CSS selectors and properties needed for correct page rendering. While the DOM represents the HTML structure, the CSSOM represents the styling, making it possible to access and change the visual appearance of elements through programming.
Render Tree
The Render Tree is created by the browser to display the web page visually. It combines the DOM and CSSOM to build a structure that contains the information needed for rendering. The tree consists of smaller elements that the browser engine processes and draws on the screen.
Different browser engines may use different terms for these elements. For example, WebKit refers to them as renderers or render objects, while Gecko calls them frames. The Render Tree follows the structure of the DOM but excludes elements that are not visible, such as those with the display:none; property, or hidden tags.
How Browsers Process and Display Web Pages
Web browsers follow a specific process to render web pages:
Building the DOM and CSSOM
When a webpage loads, the web server sends HTML, CSS, and JavaScript files to the browser. The browser engine starts by converting this data into characters and then into tokens. These tokens are further parsed into nodes.
The nodes are grouped into a tree-like structure known as the Document Object Model, which represents the HTML in a way that JavaScript can interact with. At the same time, the browser interprets the CSS and generates a comparable structure known as the CSS Object Model.
Creating the Render Tree and Displaying the Page
The browser engine then combines the DOM and CSSOM to form the Render Tree. This tree includes both the content and style information needed to display the webpage.
Next, the layout process begins. The browser utilizes the Render Tree to establish the precise placement of each visible element on the page. After the layout is complete, the browser paints the elements on the screen. This concludes the rendering process, enabling the user to view and interact with the web page.
How a Browser Renders a Webpage
Rendering a webpage is a normal activity for every browser, and the procedure is the same every time a page is shown. Below are the main steps the browser takes to turn code into a visible webpage:
- The browser receives HTML from the server and processes it into the DOM;
- It loads and parses the CSS into the CSSOM;
- The DOM and CSSOM are combined to create the Render Tree;
- Using the Render Tree, the browser calculates the layout of each element by assigning coordinates. This is done using the flow method, which processes all elements in a single pass. Unlike the older table-based layout method, this approach is more efficient;
- The browser paints the material on the screen to complete the rendering process.
Whenever the user interacts with the page or scripts make changes, the browser may need to update parts of the page. This does not always require full re-rendering. Instead, the browser uses one of the following methods:
Repaint
Repaint updates the appearance of elements without changing their layout. For example, changes in background color, visibility, or border styles trigger a repaint.
Reflow
Reflow is required when changes affect the layout or position of elements. It recalculates the layout of affected elements, including their parent and child components. Reflow can be triggered by:
- Adding, removing, or modifying DOM elements;
- Changing content, such as form input;
- Updating CSS properties;
- Adding or removing stylesheets;
- Changing the “class” attribute;
- Resizing the browser window;
- Scrolling;
- Activating pseudo-classes like :hover.
Webpage Rendering Process Explained
When you open a web page in a browser, the rendering process begins. This process comprises several processes that convert the page’s code into a visual representation on the screen. The primary steps are:
- Parsing HTML;
- Parsing CSS;
- Constructing the Rendering Tree;
- Layout;
- Painting;
- Compositing.
While the specific method varies significantly depending on the browser or device, the fundamental phases are the same.
Parsing HTML
Parsing is the process of analyzing code to understand its structure and purpose. When parsing HTML, the browser reads the code and builds a structured representation of the page’s content and elements.
When the browser gets the HTML page, it starts parsing from the top, going through each tag, attribute, and content. During this process, the browser generates a tree-like structure called the Document Object Model, which describes the page’s layout and components.
Common HTML Parsing Issues
One typical issue during HTML processing is incorrect or poorly organized HTML. Errors in the code might prevent the browser from creating a full DOM tree, resulting in missing or misplaced items and display issues. Another problem is dealing with JavaScript, which modifies the page’s content or structure after the first processing.
HTML Parsing Optimization
Optimizing HTML parsing requires writing valid and well-structured code. Tags should be properly nested, and syntax errors should be avoided. Reducing unnecessary tags or attributes also improves parsing efficiency. Using HTML validators can help detect and fix potential issues.
Parsing CSS
CSS parsing involves analyzing the stylesheet to identify which styles apply to specific elements on the page. The goal is to create a structured format of the CSS rules.
After parsing the HTML, the browser processes the CSS and generates the CSS Object Model (CSSOM), which is a tree-like structure containing all style rules. Each node in the CSSOM represents a CSS rule with its selector and properties.
Common CSS Parsing Issues
Invalid CSS code is a frequent problem during parsing. Errors or incorrect syntax can prevent the browser from creating a complete CSSOM, causing missing or incorrect styles on the page. Complex or inefficient selectors can also slow down the parsing and rendering process.
CSS Parsing Optimization
To improve CSS parsing, it is important to write clean, valid CSS with proper syntax and the correct use of braces. Avoiding overly complex selectors and unnecessary declarations helps reduce parsing time. Tools like CSS validators or preprocessors can help identify errors and optimize the stylesheet.
Constructing the Rendering Tree
The rendering tree is a hierarchical structure that integrates material from the Document Object Model (DOM) with styles from the CSS Object Model. It assists the browser in determining the final layout and visual presentation of the website.
The browser builds the rendering tree by matching each element in the DOM with the CSSOM’s related style rules. It then applies the styles, determining the layout and location of each element. The end result is a hierarchical tree containing all visible items on the page, together with their size, location, and styles.
The rendering tree only includes elements that affect the visual layout. Non-visual elements, such as <script> or <meta> tags, are ignored since they do not contribute to the page’s appearance.
Common Issues During Rendering Tree Construction
Conflicting style rules are a prevalent issue during rendering tree development. When many rules apply to the same element, the browser must determine which rule takes precedence. This can occasionally lead to unexpected layouts or display errors. Another issue is the usage of sophisticated CSS selectors, which increases processing time and may slow down rendering.
Optimization Tips
Improving the rendering tree construction starts with writing efficient and clean CSS. Avoid redundant or conflicting style rules and keep selectors simple. Reducing unnecessary HTML elements and attributes enables the browser to process the page more quickly. CSS preprocessors and layout frameworks can assist in simplifying the rendering process and improving performance.
Layout
The layout stage includes determining the size and position of each element in the rendering tree. The browser does these computations using CSS styles and DOM information. This phase ensures that all items are properly displayed on the screen and that the page appears as planned.
Painting and Compositing in Webpage Rendering
Painting is the step at which the browser uses styles and content from the rendering tree to produce the visual look of the page. This includes applying background colors, pictures, borders, and text based on the calculated styles.
After constructing the rendering tree, the browser does two primary tasks: layout and painting. The layout phase specifies the size and position of each item. Once the layout is finished, the browser begins the painting phase, which involves filling each element with the necessary styles and content.
Compositing is the final process, in which the browser mixes all painted layers to create the final visual result. This stage is responsible for stacking pieces in the proper sequence and adding transparency or blending effects as necessary.
Common Issues with Painting and Compositing
Complex or poorly optimized CSS styles can slow down the painting process and reduce page performance. Large or uncompressed images are another common issue, as they increase load times and impact the overall rendering speed.
Optimization Tips
Improving painting and compositing performance starts with clean, efficient CSS and avoiding unnecessary styles or elements. Using optimized and compressed images helps reduce load times. Tools like browser developer tools or performance profiling software can also help detect and fix issues in the painting and compositing stages.
How Browsers Improve Webpage Rendering Performance
Repainting and reflowing can both affect webpage performance, with reflowing typically causing a bigger slowdown. On devices with limited processing power, reflow may significantly reduce performance, especially if the changes require updating large parts of the page – sometimes making it as heavy as rendering the entire page again. To reduce this impact, browsers apply several optimization techniques.
One common approach is limiting the area affected by repaint or reflow. For example, if a fixed-position element changes size, reflow is limited to that element and its child elements. But if a statically positioned element changes size, the reflow affects all connected elements, causing more work for the browser.
Another optimization method involves caching changes during JavaScript execution and applying them in one pass once the script finishes running. For example, the following code triggers one repaint and one reflow:
var $body = $(‘body’);
$body.css(‘padding’, ‘1px’); // reflow, repaint
$body.css(‘color’, ‘red’); // repaint
$body.css(‘margin’, ‘2px’); // reflow, repaint
However, reading a property in the middle of these changes forces an extra reflow:
var $body = $(‘body’);
$body.css(‘padding’, ‘1px’);
$body.css(‘padding’); // reading triggers reflow
$body.css(‘color’, ‘red’);
$body.css(‘margin’, ‘2px’);
This causes two reflows instead of one, which increases the system’s stress. To avoid this, it’s recommended to group property reads together and minimize forced reflows.
In some cases, forced reflow is unavoidable — for example, when applying the same property twice with and without a transition. Consider this scenario where the margin is changed first without animation and then with a transition effect. A transition class is created in CSS:
.has-transition {
-webkit-transition: margin-left 1s ease-out;
-moz-transition: margin-left 1s ease-out;
-o-transition: margin-left 1s ease-out;
transition: margin-left 1s ease-out;
}
And the JavaScript implementation looks like this:
var $targetElem = $(‘#targetElemId’);
// Remove the transition class
$targetElem.removeClass(‘has-transition’);
// Change the property without transition
$targetElem.css(‘margin-left’, 100);
// Add the transition class back
$targetElem.addClass(‘has-transition’);
// Change the property with transition
$targetElem.css(‘margin-left’, 50);
However, the browser may cache these changes and apply them all at once, causing the transition not to work as expected. To fix this, a forced reflow is triggered to apply the first change immediately:
$(this).removeClass(‘has-transition’);
$(this).css(‘margin-left’, 100);
// Trigger forced reflow
$(this)[0].offsetHeight;
// Add the transition class back
$(this).addClass(‘has-transition’);
// Apply the final change
$(this).css(‘margin-left’, 50);
Forcing the reflow ensures the changes are applied in the correct order and the transition works as intended.
7 Practical Tips to Improve Webpage Rendering Performance
To speed up webpage rendering and improve performance across different devices and configurations, consider these seven optimization tips:
Use valid HTML and CSS with proper encoding
Include styles in the HTML file and place scripts at the end of the <body> tag to avoid blocking the page load.
Choose simple CSS selectors.
Avoid overly complex selectors and minimize nesting. From fastest to slowest, the general order of selector performance is:
- ID selectors (#id)
- Class selectors (.class)
- Tag selectors (div)
- Adjacent sibling selectors (a + i)
- Parent selectors (ul > li)
- Universal selectors (*)
- Attribute selectors (input[type=”text”])
- Pseudo-classes and pseudo-elements (a:hover)
Limit DOM manipulation and use caching where possible
Reduce direct changes to the DOM. For complex updates, build the element in memory first and then insert it into the DOM. Reuse cached properties or objects when needed.
Follow best practices with jQuery selectors
Optimize jQuery usage by writing efficient selectors to avoid unnecessary performance issues.
Use the .class attribute for style changes
Applying styles by changing the class is more efficient. The further down the DOM tree the change happens, the less impact it has on rendering.
Animate fixed or absolute positioned elements
Animating these elements uses fewer resources compared to static elements, making the animation process smoother.
Disable complex :hover animations during scrolling
This helps reduce system load and keeps the page responsive while users scroll.
Conclusion
Webpage rendering plays an important role in how websites perform and display content to users. Every stage of the process, from parsing HTML and CSS to constructing the rendering tree and applying styles, directly impacts page speed and user experience. Optimizing each step helps reduce delays, prevents display issues, and improves overall performance. By following recommended practices, websites can maintain efficient rendering, support smooth interactions, and perform consistently across different browsers and devices.
