How do you keep parents of floated elements from collapsing using HTML?

Although elements like <div>s normally grow to fit their contents, using the float property can cause a startling problem for CSS newbies: If floated elements have non-floated parent elements, the parent will collapse.

For example:

<div style="float: left;">
  <div style="float: left;">Div 1</div>
  <div style="float: left;">Div 2</div>
</div>