How HTML, CSS and JS works together

To understand how HTML, CSS, and JavaScript work together in a web browser, let's break down the process step by step. This article will cover the roles of each technology, how the browser interprets them, and what happens behind the scenes

The Role of HTML, CSS, and JavaScript

1. HTML (HyperText Markup Language):

   • HTML provides the structure and content of a webpage.
   • It defines elements like headings, paragraphs, images, links, buttons, and other components using tags (<div>, <p>, <img>, etc.).

2. CSS (Cascading Style Sheets):

   • CSS is responsible for the presentation and styling of the webpage.
   • It controls layout, colors, fonts, spacing, and other visual aspects of the HTML elements.

3. JavaScript:

   • JavaScript adds interactivity and dynamic behavior to the webpage.
   • It can manipulate the DOM (Document Object Model), handle events (e.g., clicks, form submissions), fetch data from servers, and more.

How the Browser Interprets the Code

When you open a webpage in a browser, the following steps occur:

1. Fetching the Resources

• The browser sends an HTTP/HTTPS request to the server hosting the webpage.
• The server responds with the HTML file, which may also reference external CSS and JavaScript files (via <link> and <script> tags).

2. Parsing the HTML

• The browser starts parsing the HTML document line by line.
• During parsing:
  • It builds the DOM Tree, which is a hierarchical representation of the HTML elements.
  • If it encounters a <link> tag for CSS or a <script> tag for JavaScript, it pauses HTML parsing to fetch and process those resources.

3. Loading and Parsing CSS

• When the browser encounters a CSS file (linked via <link>), it downloads and parses the CSS rules.
• It creates the CSSOM (CSS Object Model), which is a tree-like structure representing the styles applied to each element in the DOM.
• The browser combines the DOM and CSSOM to create the Render Tree, which determines what is visible on the screen and how it should look.

4. Rendering the Page

• Using the Render Tree, the browser performs layout calculations (determining the position and size of each element) and then paints the elements on the screen.
• This process ensures that the page is displayed correctly according to the HTML structure and CSS styles.

5. Executing JavaScript

• When the browser encounters a <script> tag, it executes the JavaScript code.
• JavaScript can:
  • Manipulate the DOM (e.g., add, remove, or modify elements).
  • Change styles dynamically (e.g., toggle classes, update inline styles).
  • Handle user interactions (e.g., respond to button clicks, form submissions).
  • Fetch additional data from servers using APIs (e.g., fetch or XMLHttpRequest).

Important Note: JavaScript execution can block HTML parsing unless the <script> tag includes the async or defer attribute. These attributes allow the browser to continue parsing the HTML while loading or executing the script.

What Happens in the Background

1. DOM Construction:

   • The browser converts the HTML into a tree-like structure called the DOM.
   • Each HTML element becomes a node in the DOM tree.

2. CSSOM Construction:

   • The browser processes the CSS rules and creates the CSSOM, which maps styles to DOM nodes.

3. Render Tree Creation:

   • The browser combines the DOM and CSSOM to create the Render Tree.
   • Only visible elements are included in the Render Tree (e.g., elements with display: none are excluded).

4. Layout and Painting:

   • The browser calculates the layout (position and size of elements).
   • It then paints the elements on the screen, applying the styles defined in the CSS.

5. JavaScript Execution:

   • JavaScript runs in the browser's JavaScript engine (e.g., V8 in Chrome).
   • It can modify the DOM and CSSOM, triggering reflows (recalculating layout) and repaints (redrawing elements).

The Role of the Compiler/Interpreter

• HTML and CSS:

  • HTML and CSS are not compiled but interpreted directly by the browser.
  • The browser's rendering engine (e.g., Blink in Chrome, Gecko in Firefox) parses and processes these languages.

• JavaScript:

  • JavaScript is interpreted by the browser's JavaScript engine.
  • Modern JavaScript engines use Just-In-Time (JIT) compilation to optimize performance:
    • The engine first parses the JavaScript code into an Abstract Syntax Tree (AST).
    • It then compiles the code into machine code for faster execution.
    • The engine continuously optimizes the code during runtime based on usage patterns.

Summary of the Workflow

1. Request and Fetch:

   • The browser requests the HTML file from the server and fetches any linked CSS and JavaScript files.

2. Parse HTML:

   • The browser parses the HTML and builds the DOM tree.

3. Load and Parse CSS:

   • The browser loads and parses the CSS, creating the CSSOM.

4. Create Render Tree:

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

5. Layout and Paint:

   • The browser calculates the layout and paints the elements on the screen.

6. Execute JavaScript:

   • The browser executes JavaScript, which can modify the DOM, CSSOM, and trigger reflows/repaints.

7. Continuous Updates:

   • The browser continuously updates the page as needed (e.g., in response to user interactions or data fetched from APIs).

Key Takeaways

• HTML provides the structure, CSS handles the styling, and JavaScript adds interactivity.
• The browser parses HTML to build the DOM, parses CSS to build the CSSOM, and combines them to create the Render Tree.
• JavaScript is interpreted and executed by the browser's JavaScript engine, which uses JIT compilation for optimization.
• The browser performs layout and painting to display the webpage, and JavaScript can dynamically modify the page.

Now you can understand how HTML, CSS and JS works together to create a web page.