id

An "id" attribute in HTML is used to uniquely identify an element on a web page. It is similar to the "class" attribute in that it can be used to select and style the element with CSS. However, while a class can be assigned to multiple elements, an id can only be used once on a page.

Here is an example of a "div" element with an id:

<div id="header">
 <h1>Welcome to my website</h1>
</div>

In this example, the "div" element has an id of "header". This id can be used to select the div with CSS and apply styles to it:

#header {
 background-color: #f2f2f2;
 text-align: center;
}

This CSS will change the background color of the div to #f2f2f2 and center the text inside the div.

An id is used to select a unique element, therefore it's important that the id is used only once in the page, otherwise it will break the expected behavior.

As you can see, you can use both HTML and CSS to create and style elements on your web pages, and ids can be used to select and style unique elements.