


Understanding the HTML `id` and `class` attributes for styling and scripting.
Jul 02, 2025 pm 03:26 PMId is a unique identifier used to locate a single element; class is used to define the same style or behavior of multiple elements. 1. The id must be unique in the entire page, suitable for precise control and scripting operations on specific elements, with high priority in CSS; 2. The class can be reused, suitable for unified setting of styles and interactive logic of multiple elements; 3. In actual development, id and class are often used in combination, which not only ensures the reusability of the styles, but also achieves precise control.
HTML's id
and class
attributes are the two most basic and commonly used features in web development, especially in style design (CSS) and script operations (JavaScript). Although they look similar, they have obvious differences in usage and function.

What is id
? When should I use it?
id
is used to uniquely identify an element in the page. Each id
must be unique throughout the page and cannot be repeated. This makes it ideal for positioning specific elements, such as the page's navigation bar, main title, or a specific button.

Recommended usage:
- When you need to style or script the "unique" elements in the page, use
id
first. - Get elements through
document.getElementById()
in JavaScript, fast and accurate. -
id
have higher priority in CSS and are easy to overwrite other style rules, so be careful to avoid style conflicts.
For example:

<div id="main-header">This is the main title of the page</div>
This main-header
can only appear once. If it is reused, although the browser will not report an error, it is incorrect in terms of semantics and specifications.
What is class
again? What is it more suitable for?
class
is used to label multiple elements with the same "label" to indicate that they have a common set of styles or behaviors. Unlike id
, the same class
can be reused on multiple elements.
Recommended usage:
- If multiple elements need to share the same style or interactive logic, use
class
. - In CSS, styles are defined through the
.classname
selector. In JavaScript, you can usedocument.getElementsByClassName()
ordocument.querySelectorAll()
to operate a set of elements. - An element can have multiple
class
, separated by spaces.
For example:
<p class="highlight">This is a highlight paragraph. </p> <p class="highlight">This is another highlight paragraph. </p>
In this way, you can set the styles of all highlight
classes uniformly, such as background color, font bold, etc.
How to use id
and class
in actual development?
Sometimes you will see an element with both id
and class
, which is actually very common and reasonable. For example, a button may be the only login button in the entire page (using id
), but it belongs to a common style button type (using class
).
<button id="login-button" class="btn primary">Login</button>
In this example:
-
btn
may be a universal button style -
primary
may be a style variant that emphasizes buttons -
login-button
is a unique identifier, which is convenient for JS operation.
This method not only maintains the reusability of the style, but also allows precise control of specific elements.
Let's summarize
-
id
is unique, suitable for positioning and manipulating individual elements -
class
repeatable, suitable for style definition and batch operations - Use it frequently in actual development to give full play to their respective advantages
Understanding the difference and usage of these two attributes is the basis for writing web pages with clear structure, controllable style, and easy to maintain scripts. Basically that's it.
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