How to style the UL list to a single line

HTML code:

<ul class="list">
  <li>Item 1</li>
  <li>Item 2</li>
  <li>Item 3</li>
</ul>

CSS code:

ul.list li{
  width: auto;
  float: left;
}

ul li{
  display: inline;
}

For more see the basic list options and a basic horizontal list at listamatic. (thanks to Daniel Straight below for the links).

Also, as pointed out in the comments, you probably want styling on the ul and whatever elements go inside the li's and the li's themselves to get things to look nice.

In modern browsers you can do the following (CSS3 compliant)

ul
{
  display:flex;  
  list-style:none;
}

<ul>
  <li><a href="">Item1</a></li>
  <li><a href="">Item2</a></li>
  <li><a href="">Item3</a></li>
</ul>