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How does a browser determine what elements match a CSS selector ?

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When a browser loads a web page, it first parses the HTML and creates a Document Object Model (DOM) tree. The DOM tree is a hierarchical representation of the HTML elements on a web page. Each element in the DOM tree is represented as a node, and the relationships between nodes represent the structure of the HTML document.

Once the DOM tree and the CSS object model are created, the browser combines them to determine how the web page should be rendered. The browser starts at the root of the DOM tree and applies the styles in the CSS object model to the corresponding elements in the DOM tree.

The browser uses a matching process to determine which styles should be applied to each element. The matching process starts at the root of the DOM tree and compares each element to the CSS selectors in the CSS object model. If an element matches a selector, the styles associated with that selector are applied to the element.

The matching process begins with the most specific selector and works its way down to the least specific. For example, if a selector is #id, it will be more specific than .class and will be applied first. If there are multiple selectors with the same specificity, then the order in which the selectors appear in the stylesheet determines which one will be applied.

The browser uses different algorithms to match the selectors and elements. The most common algorithm is the “matching by tag name” algorithm, where the browser looks at the tag name of the element and compares it to the tag name in the selector. For example, if the selector is p, the browser will check every p element in the DOM tree.

Another common algorithm is the “matching by class and ID” algorithm, where the browser looks at the class and ID attributes of the element and compares them to the class and ID selectors. For example, if the selector is .highlight, the browser will check every element with a class attribute of “highlight”.

 

Example 1: 

HTML




<!DOCTYPE html>
<html>
  
<head>
    <style>
        .container h2 {
            color: green;
        }
    </style>
</head>
  
<body>
    <div class="container">
        <h2>Geeks\forGeeks</h2>
        <h2>This is Heading!</h2>
    </div>
</body>
  
</html>


Output: 

How does a browser determine what elements match a CSS selector?

How does a browser determine what elements match a CSS selector?

Explanation: In this example, the browser will use the “matching by class and ID” algorithm to match the ‘h2’ elements inside the ‘.container’ element to the ‘.container h2’ CSS selector. The browser will apply the ‘color: green’ style to the headings.

Example 2: 

HTML




<!DOCTYPE html>
<html>
  
<head>
    <style>
        .highlight {
            background-color: green;
            color: white;
        }
    </style>
</head>
  
<body>
    <div class="container">
        <h1 style="color:green">
              GeeksforGeeks
          </h1>
        <h2 class="highlight">
              This is a highlighted Heading!
          </h2>
    </div>
</body>
  
</html>


Output: This example will change the background color of the heading elements that contain the class “highlighted” in their text to green.

How does a browser determine what elements match a CSS selector?

How does a browser determine what elements match a CSS selector?

Explanation: In this example, the browser will use the “matching by class and ID” algorithm to match the ‘h2’ element with class “highlight” to the ‘.highlight’ CSS selector. The browser will apply the ‘background-color: green’ style to the paragraph.

In both examples, the browser starts at the root of the DOM tree. It compares each element to the CSS selectors in the CSS object model, and if an element matches a selector, the styles associated with that selector are applied to the element.

Conclusion: In conclusion, determining what elements match a CSS selector is a crucial step in the process of rendering a web page. The browser uses the Document Object Model (DOM) tree and the CSS object model to determine which styles should be applied to each element on the page. It starts at the root of the DOM tree and compares each element to the CSS selectors in the CSS object model using different matching algorithms. This process allows the browser to separate the presentation of a web page from its structure and content, making it easier to maintain and update a website.



Last Updated : 12 Feb, 2023
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