The role of id and class in CSS is different, and the usage scenarios are also different. 1. Id is a unique identifier, used for style design of a single unique element, such as #main-content; 2. class can be reused and is suitable for multiple elements to apply the same style, such as .button; 3. The selector of id is higher than class, which will affect style overwrite; 4. Use id in HTML to use id, class to use class="name"; 5. Use #name to select id in CSS, and use .name to select id; 6. It is recommended to avoid overuse of id for style design, because it is not easy to cover; 7. Use id and class to use id at the same time; 8. If you need to operate elements through JavaScript, it is more convenient to use id.

When you're styling HTML elements with CSS, two of the most commonly used selectors are id and class . While they both target elements on the page, they serve different purposes and have distinct behaviors.

One Unique ID, Many Reusable Classes

An id is meant to be unique within a page — you should only use one id per element, and that same id shouldn't appear on more than one element. This makes id perfect for styling elements that only appear once, like a header or a main navigation bar.

A class , on the other hand, can be used on multiple elements. That makes it ideal when you want to apply the same style to several different elements. For example, if you have multiple buttons that should look the same, giving them the same class makes your CSS cleaner and more maintained.

Here's a quick comparison:

• Use id for one-of-a-kind elements.
• Use class for repeated styling across multiple elements.

Specific Matters

One key difference that often trips people up is specific . In CSS, when two rules apply to the same element, the one with higher specific wins.

An id selector has higher specificity than a class selector. That means if you have conflicting styles — say, a class sets the text color to blue and an id sets it to red — the id rule will take precedence.

This can be helpful when you want to override general styles for a specific element, but it can also lead to hard-to-debug issues if you're not careful.

HTML and CSS Syntax Differences

The syntax for using id and class is slightly different in both HTML and CSS.

In HTML:

• id is written as id="header"
• class is written as class="button"

In CSS:

• id is selected with a hash: #header { ... }
• class is selected with a dot: .button { ... }

It's a small difference, but it's important to get right. Using the wrong symbol in your CSS will mean your styles don't apply at all.

When to Use Which?

Here are a few practical guidelines:

• Use id when targeting a single, unique element (like #main-content or #sidebar )
• Use class when applying styles to multiple elements (like .highlight or .nav-link )
• Avoid overusing id for styling — it can make your CSS harder to override later
• You can use both id and class on the same element if needed

Also, JavaScript often uses id to target specific elements, so if you plan to manipulate an element with JS, using an id might make things easier.

Basically that's it.

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