How Does a Web Browser Load and Execute a Web Page?

Load and Execution Sequence of a Web Page

When a web page is loaded, the browser goes through a series of steps to process and render the page. The following is a simplified overview of this process:

1. HTML Parsing

The browser downloads the HTML document and begins parsing it. This involves tokenizing the HTML code, creating a DOM tree, and checking for syntax errors.

2. CSS Parsing

As the HTML is parsed, the browser also identifies and loads any external CSS files. These CSS files are parsed into a set of style rules, which are then applied to the DOM.

3. JavaScript Execution

External JavaScript files are downloaded and executed in the order they appear in the HTML. Inline JavaScript is executed as it is encountered during HTML parsing.

4. Resource Loading

Other resources, such as images, fonts, and media files, are downloaded in parallel and cached for future use.

5. DOM Manipulation and Event Handling

After all external resources have been loaded, the browser creates the final DOM tree and applies the CSS styles. Event listeners are bound to HTML elements, allowing users to interact with the page.

6. Image Replacement

In your example, kkk.png will replace abc.jpg as the source for the image with the id "img." This is because the code in $(document).ready() executes after all external resources have been loaded and the DOM is ready.

7. Image Rendering

The replaced image is downloaded and rendered on the page.

Differences in Browsers

While the general load and execution sequence is consistent across browsers, there may be minor variations in implementation. For instance, some browsers may prioritize CSS parsing over JavaScript execution or implement optimizations such as lazy loading of images.

Note on Parallel Execution

While CSS and resource loading can occur in parallel, the execution of JavaScript follows a single-threaded model. This means that the browser executes JavaScript code one statement at a time, and the execution of external scripts blocks the parsing of the HTML document.

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