A ID selector selects all HTML tags that matches the id name defined in the tag attribute of id="id_name". ID selectors are specified with a leading hash like (#id_name). For example, the following CSS definition uses an ID selector:

/* set text to red to all tags with id="onSale" */

#onSale {color: red}

If you apply the above CSS definition to the following HTML document, you will get two blocks in red, one from the <p> tag and one from the <pre> tag:

<p>Normal paragraph...</p>

<p id="onSale">Special paragraph...</p>

<pre>Normal pre-formatted text...</pre>

<pre id="onSale">Special pre-formatted text...</pre>