The id attribute

id sets a unique identifier for an element within the whole document — used by CSS, JavaScript, anchor links (#id), and label.

Applies to

global

Values

Any string with no spaces (the modern spec allows almost any character except spaces).

Example

<h2 id="pricing">Pricing</h2>
<a href="#pricing">Jump to pricing</a>

Edge cases

id must be unique across the whole page — duplicates break #anchor behavior, label for, and querySelector('#...').
An id starting with a digit is technically valid in HTML5 but can cause issues in CSS selectors without escaping.

Accessibility

id is the foundation for label for, aria-labelledby, and aria-describedby.

Related tools

The <label> tag

The for attribute of <label>, linking a caption to an input.

The class attribute

The global class attribute, used for CSS styling and for querySelector selection.