Author: Vincent Ortiz
Published: 10/23/2025

What is the Essential Difference Between Class and ID in HTML and CSS?

Q: What’s the best practice when using both?

There’s no single rule — it depends on your project. Typically, use classes for general styling and IDs for unique behavior or exceptions. Keep your code clean by avoiding unnecessary overlap between the two.

When you begin the journey of web development, you quickly realize that the visual appeal and structure of your site relies heavily on CSS (Cascading Style Sheets). CSS is the language that styles your website elements, but to do that, you need a reliable way to select those elements. This brings us to the two most fundamental selectors: the Class and the ID.

While both are used to hook into an HTML element for styling or JavaScript manipulation, they are designed for completely different jobs. Understanding their distinct roles and rules is essential for writing clean, scalable, and bug-free code.

How Do I Use the Class Selector (.) to Style Groups of Elements?

The class selector is your tool for reusability, flexibility, and consistency.

Imagine you’re designing a website with dozens of cards, buttons, or informational boxes. You don't want to write the same size, border, and font rules for each one individually. This is where the class shines. It lets you create a style blueprint once, and apply it to an unlimited number of elements across your entire website.

The Class Rules: The Power of Reusability

To implement a class, you simply add the class="" attribute inside the opening HTML tag. To style a group of elements the same way, you give them all the exact same class name.

Goal: Group styling, shared appearance, and component design.
Best Practice: Always favor classes over IDs for general styling.
Key Feature: You can apply multiple classes to a single element (e.g., <button class="btn primary-color large-text">).

Key Naming Rule: When naming a class, you cannot use spaces . Whatever elements you want to style identically must share the exact same class name.

Let's use the example of three <div> elements that need the same dimensions. We'll give them the class "box-size":

How Do I Target the Class in CSS?

To actually style the elements in your CSS file, you target the class name by preceding it with a period (.).

After applying this CSS, all three divs are styled the exact same way, instantly inheriting the size and color of the class. If you decide later that the boxes should be 300px wide instead of 500px, you only have to edit that single line of code, and all 3 elements with that class are updated simultaneously.

2. When Should I Use the ID Attribute (#) for a Single, Unique Element?

If a class is for groups, the ID attribute is reserved for unique elements—the single, most important structural pieces of your page that only appear once.

Think of an ID as a unique identifier for a major structural component, like a main site navigation bar, a footer, or a primary banner section. It must only be used on one element per HTML page.

ID Priority: Why Does ID Override a Class?

The main functional difference is that if an element has both a class and an ID, and you define the same property in both (like background-color or padding), the ID will override the class.

How to Use the ID Selector for Edge Cases

In CSS, the ID is targeted using a hash/pound sign (`#`). In HTML, you apply the ID by simply putting: `id="idname"`.

Let's use three different ID's to change the background color of the three divs from the class example, overriding the shared background-color: red; set by the class:

If a class is for groups, the ID attribute is reserved for unique elements—the single, most important structural pieces of your page that only appear once.

ID Priority: Why Does ID Override a Class?

How to Use the ID Selector for Edge Cases

In CSS, the ID is targeted using a hash/pound sign (`#`). In HTML, you apply the ID by simply putting: `id="idname"`.

Let's use three different ID's to change the background color of the three divs from the class example, overriding the shared background-color: red; set by the class:

Now, we style each one uniquely using the # selector in CSS:

The divs still keep their dimensions from the class (.box-size), but the unique IDs overrode the shared background-color for each one. This is the only way to apply a unique style to one element without writing inline CSS.

Why is the "Unique ID" Rule Non-Negotiable for JavaScript?

This rule is about more than just styling; it's a foundational standard for JavaScript and web behavior.

When you write JavaScript to make your page dynamic (like making a button disappear or changing text based on a user action), you rely heavily on the element’s unique identifier:

For Classes: You use document.querySelectorAll('.box-size') to grab a group of elements.
For IDs: You use the dedicated, highly reliable function document.getElementById('pink') to grab a single element.

If you mistakenly use the same ID on multiple elements, JavaScript will only target the first one it finds, and your dynamic code will break or behave unpredictably for the others. To write clean, reliable, and functional code, you must treat the ID attribute as a unique serial number.

Final Thoughts and Key Best Practices

Hopefully, you now have a comprehensive understanding of ID and Class in HTML and CSS and how they are used:

Classes are the workhorse of modern CSS. They make it easy to create reusable components and styles that ensure consistency across your entire website.
IDs are the specialists. They allow you to handle highly specific "edge cases" for one particular element or provide a unique, reliable target for JavaScript functions.

FAQ

Can an ID be used to style multiple elements like a class?

Technically, yes — but it’s bad practice. IDs are meant to be unique and used only once per page. Reusing an ID can cause JavaScript conflicts and unexpected styling issues, since scripts often target a specific element by its ID.

Are classes and IDs only for styling?

No. While they’re often used for CSS styling, both are also important for JavaScript functionality. Classes are great for targeting and updating groups of elements, while IDs are ideal when you need to manipulate a single, specific element—for example, updating content or adding interactivity dynamically.

Which has higher priority in CSS: a class or an ID?

An ID always overrides a class when both target the same property. For example, if both define a color, the style from the ID will take effect because IDs have higher specificity in CSS.

Do I need both a class and an ID?

Usually, no. You can use just one depending on your needs. Use a class when styling or scripting multiple elements, and an ID for targeting one unique element.

What’s the best practice when using both?

There’s no single rule—it depends on your project. Typically, use classes for general styling and IDs for unique behavior or exceptions. Keep your code clean by avoiding unnecessary overlap between the two.