What is the difference between the ‘id’ attribute and the ‘class’ attribute of HTML elements?

Question

Multiple elements in HTML can have the same class value, whereas a value of the id attribute of one element cannot be associated with another HTML element.

Answer 1

The id and class attributes are used to specify the style and behavior of HTML elements, but they have different purposes and behaviors:

1. id Attribute: The id attribute is used to identify a unique HTML element on a web page. It should be unique within the entire HTML document. The id attribute is often used in JavaScript and CSS to style and manipulate specific HTML elements. For example, you might use an id attribute to identify a specific section of a webpage that you want to highlight with a particular color or font size. An id attribute can be used only once within a single HTML document.
2. class Attribute: The class attribute is used to specify a group of HTML elements that share the same style or behavior. Multiple elements in HTML can have the same class value, and a single HTML element can have multiple class values. The class attribute is often used in CSS to apply styles to groups of HTML elements. For example, you might use a class attribute to specify a set of links that all share the same background color or font size.

In summary, the main difference between the id attribute and the class attribute is that the id attribute is used to identify a unique HTML element, while the class attribute is used to specify a group of elements that share the same style or behavior.

Answer 2

Hello, the answer by Ryze S sums up the fundamental differences between the two. I want to supplement this: When using CSS to style the HTML page, the style specified on an id supersedes the style specified on a class. For example, if an element has both an id and class, and if you specify conflicting styles in that id and class, then no matter where (up or down the page) on the CSS stylesheet the styles are specified, the element will be styled the way the style for the id has been defined.

This happens simply because, by definition, class is a generic identifier for an element while the id is specific. Think of an equivalent scenario in real life. You will have some qualities as a human being (class), but your specific qualities (id, say your name or the human entity that's you) will supersede.

Therefore, later down the flow in the stylesheet if you try to modify based on some combined scenario (for example, for responsive design, if you define the style for the class differently for mobile view), yet that may not be applied just because the style of the id still maintains the priority. Therefore, it would be very difficult to modify styles given to an id by any other modifier later in the code flow. That is the reason why it is recommended not to use id for styling the page.

On the other hand when you are writing the JavaScript code and want to add an event or interact with an element, then id is a better target to specifically select an element and act on it.

Answer 3

Hi,

A very quick Google search would bring up this reponse:

Difference between id and class attribute: The only difference between them is that “id” is unique in a page and can only apply to at most one element, while “class” selector can apply to multiple elements. 

Difference between an id and class in HTML? - GeeksforGeeks

https://www.geeksforgeeks.org

So, for a real world example that might be easier to understand:

I have 2 cats, Twinkie and Inkie. The class is cats, the ids are Twinkie and Inkie. At dinner time, if I feed the class, both cats get fed and purr. If I only feed the id="Twinkie", then id="Inkie" gets mad and hisses..

Both id and class are used to identify the element you wish to manipuate.