When should you use id, and when class in HTML?

id is used as a unique identifier for an element on the page. In a document, only one element can have the same id value. It is often used for linking labels to input fields (<label for="...">) or for anchors (<a href="#...">). It can also be useful for quick access to an element via JavaScript (document.getElementById(...)).

class is used for grouping elements with common styles or behavior. Multiple elements can share the same class. This is the primary way to apply CSS styles. The same element can have multiple classes separated by spaces.

Usage	Identifier (id)	Class (class)
Uniqueness	Strictly unique on the page	Can be repeated
Number per element	One	Multiple allowed
Main use	Unique link, anchor, JavaScript access	Grouping for styles and JS
CSS selector	#	.