What is the difference between class and ID in HTML?

 

When building a website, structuring your HTML elements properly is crucial for clean, maintainable, and scalable code. 

Two of the most commonly used attributes for targeting and styling elements are class and ID. While both serve the purpose of identifying elements, they do so in very different ways.

 Understanding these differences not only improves your code's readability but also enhances website performance and styling precision.

In this comprehensive guide, we’ll dive deep into what class and ID are in HTML, their key differences, and best practices for using and naming them. 

We'll also explore CSS specificity, when one overrides the other, and real-world examples to help you apply these concepts effectively.

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Table of Contents

1. What is a Class in HTML?
2. What is an ID in HTML?
3. What is the Difference Between Class and ID in HTML?
4. Is Class Stronger Than ID?
5. Does ID Override Class in HTML?
6. How to Name Class and ID in HTML?
7. When to Use Class vs. ID in HTML
8. Common Mistakes to Avoid
9. Conclusion

 

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1. What is a Class in HTML?

A class in HTML is an attribute that allows you to group multiple elements together under a common name, making it easier to apply the same styles or behaviors to them. Classes are one of the cornerstones of CSS styling and JavaScript manipulation.

Example:

<p class="highlight">This paragraph is highlighted.</p>

<div class="highlight">This div is also highlighted.</div>

<button class="highlight">This button is highlighted too!</button>

In this example, all three elements share the highlight class, meaning you can apply the same CSS style to them with a single rule.

Use Cases for Classes:

• Applying the same style to multiple elements (e.g., all buttons on a page).
• Grouping elements for JavaScript manipulation (e.g., toggling visibility).
• Adding multiple classes to a single element for more flexible styling.

Example of Multiple Classes:

<button class="btn primary large">Click Me</button>

 

Here, the button has three classes: btn, primary, and large, each potentially adding different styles.

Key Points:

• Reusable: The same class can be assigned to multiple elements on the same page.
• Flexible: You can combine multiple classes to create complex styling.
• Selector in CSS: Use a period (.) followed by the class name (e.g., .highlight).

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2. What is an ID in HTML?

An ID is a unique identifier assigned to a single element in an HTML document. Unlike classes, IDs should only be used once per page. They are ideal for targeting specific elements, especially when you need to apply unique styles or behaviors.

Example:

<p id="unique-text">This paragraph has a unique ID.</p>

 

Use Cases for IDs:

• Targeting specific elements for unique styling.
• Creating anchors for navigation links (e.g., jumping to a section of the page).
• Manipulating specific elements with JavaScript.

Anchor Example:

<a href="#contact-section">Go to Contact Section</a>

<section id="contact-section">
  <h2>Contact Us</h2>
</section>

 

Clicking the link will scroll the page to the Contact Us section.

Key Points:

• Unique: An ID should be unique within a page.
• Specific: IDs are perfect for elements that require unique styling or behavior.
• Selector in CSS: Use a hash (#) followed by the ID name (e.g., #unique-text).

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3. What is the Difference Between Class and ID in HTML?

While both class and ID are used to identify elements in HTML, they serve different purposes and have distinct behaviors.

• Reusability: A class can be applied to multiple elements, making it useful for styling and grouping similar elements. On the other hand, an ID must be unique within a single page, meaning it can only be assigned to one element.

• CSS Selector: When styling elements in CSS, a class is referenced using a period (.), such as .class-name, whereas an ID is referenced using a hash (#), like #id-name.

• JavaScript Access: In JavaScript, you can select elements with a specific class using document.getElementsByClassName(), which returns a collection of matching elements. In contrast, document.getElementById() selects a single element based on its unique ID.

• Purpose: Classes are primarily used for grouping multiple elements with similar styles or behaviors. IDs, however, are used to target a single, unique element that requires distinct styling or scripting.

• Specificity: IDs have higher specificity than classes in CSS. If both a class and an ID define conflicting styles for the same element, the ID’s style will take precedence.

• Performance: Since classes can apply to multiple elements, they require more processing when accessed via JavaScript or CSS. IDs, being unique, allow for faster selection and manipulation, making them slightly more efficient in performance.

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4. Is Class Stronger Than ID?

In terms of CSS specificity, ID is stronger than class. CSS specificity is a system that determines which style rules apply to an element when there are conflicting rules. The more specific the selector, the higher its priority.

CSS Specificity Hierarchy (from lowest to highest):

1. Element selectors (e.g., p, div, h1).
2. Class selectors (e.g., .highlight).
3. Attribute selectors (e.g., [type="text"]), pseudo-classes (e.g., :hover).
4. ID selectors (e.g., #unique-text).
5. Inline styles (e.g., style="color: red;").
6. !important declarations (overrides all other styles).

Example:

<p class="text" id="highlighted-text">This text has both a class and an ID.</p>

 

CSS:

.text {
 color: blue;
}

#highlighted-text {
  color: red;
}

 

Result: The text will appear red because the ID selector has higher specificity than the class selector.

 

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5. Does ID Override Class in HTML?

Yes, when both an ID and a class are applied to the same element, the ID will override the class styles in CSS. However, this only affects styling; it doesn’t impact the HTML structure or the behavior of the elements unless you’re using JavaScript.

JavaScript Example:

document.getElementById("highlighted-text").style.fontSize = "20px";

document.getElementsByClassName("text")[0].style.color = "green";

 

Here, the ID targets the specific paragraph, while the class targets the first element with the text class.

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6. How to Name Class and ID in HTML?

Proper naming conventions make your code easier to read, maintain, and debug. Here are some best practices:

Best Practices for Naming Classes and IDs:

1. Be Descriptive: Use names that clearly describe the element's purpose (e.g., .navbar, #footer-logo).
2. Use Hyphens for Classes: Hyphens (-) improve readability (e.g., .main-content, .btn-primary).
3. Use CamelCase for IDs: CamelCase is often used for IDs (e.g., #headerTitle, #contactForm).
4. Avoid Reserved Words: Don’t use keywords like class, id, or submit as names.
5. Consistency is Key: Stick to one naming convention throughout your project.

Examples of Good Naming:

<div class="product-card">

  <h2 id="productTitle">Product Name</h2>
  <button class="btn btn-add-to-cart">Add to Cart</button>

</div>

 

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7. When to Use Class vs. ID in HTML

Knowing when to use class and ID is crucial for efficient HTML and CSS management.

Use Classes When:

• You need to apply the same styles to multiple elements (e.g., .button, .card).
• You’re grouping elements for JavaScript (e.g., toggling multiple modals).

Use IDs When:

• You need to style or manipulate a unique element (e.g., #main-header).
• Creating anchor links for navigation (e.g., #about-section).

 

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8. Common Mistakes to Avoid

1. Using IDs for Multiple Elements: IDs should be unique. Reusing an ID can cause unexpected behavior in CSS and JavaScript.
2. Overusing IDs for Styling: Since IDs have higher specificity, overusing them can make your CSS difficult to manage.
3. Mixing Naming Conventions: Be consistent with either hyphens or camelCase but avoid mixing styles in the same project.

 

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Conclusion

Understanding the differences between class and ID in HTML helps you write clean, maintainable, and efficient code. While classes offer flexibility and are great for grouping elements, IDs provide unique targeting for specific elements. Knowing when and how to use them, along with proper naming conventions, will enhance your web development skills and lead to better-performing websites.

By mastering these concepts, you’ll not only improve your HTML and CSS workflow but also create more organized and scalable web projects.