CSS id selector is used to select the HTML element using the ID attribute to apply a style to it. This id element is distinctive always inside the page, and hence it is preferred for selecting a distinct, unique HTML element. When you want to apply the style to only one HTML tag at a time on a page, then you can use the ID selector.

Id selector can be implemented by writing it with the hash (#) character, trailing with your desired element's id.

Syntax:

#id_name{
/* declarations */
}

Here is an example of an Id selector implemented on a particular div tag:

CSS Id Selector Example

Example:

<!DOCTYPE html>
<html>

    <head>
        <style>
            #mainframe {margin: auto, max-width: 1200px} 
        </style>
    </head>

    <body>
        <!-- On this element, the ID selector class will be applied. -->
        <div id="mainframe">
            <!-- This is the main container of page. -->
            <p>This paragraph is not assigned with id selector and hence no affect on it.</p>
        </div>
    </body>

</html>