Up until recently I didn’t realize you could style horizontal rules (<hr />). Of course you could use a div to do the same thing, but using a horizontal rule for its intended use is nice (and easier than using a div).

Here’s the CSS I used most recently on a website I’m making to customize the horizontal rule.

hr {
    width: 90%;
    border: 0;
    height: 1px;
    background: #000;
    opacity: 0.2;
}

Of course you can also name the style and apply it to the horizontal rule.

// CSS
hr.custom {
    width: 90%;
    border: 0;
    height: 1px;
    background: #000;
    opacity: 0.2;
}

// HTML
<hr class="custom" />

You can also do things like gradients, blurs, etc.