We've used a number of HTML elements as CSS selectors so far: we've styled the HTML tags <h1></h1> with the CSS selector h1, <p></p> with p, and so on.
You may have guessed this, but if not, we'll say it outright: any HTML element can be a CSS selector! You can modify <ul>s, <table>s, and even the entire <body> by selecting ul, table, and body, respectively.