Equal size columns with pure CSS. Uses bottom-padding-margin-trick where the overstanding elements will be cutted to proper size by overflow:hidden. From http://stackoverflow.com/questions/1205159/html-css-making-two-floating-divs-the-same-height

equal_columns.css

#container {
  overflow: hidden;
      width: 100%;
}
#left-col {
  float: left;
  width: 50%;
  background-color: orange;
  padding-bottom: 500em;
  margin-bottom: -500em;
}
#right-col {
  float: left;
  width: 50%;
  margin-right: -1px; /* Thank you IE */
  border-left: 1px solid black;
  background-color: red;
  padding-bottom: 500em;
  margin-bottom: -500em;
}

equal_columns.html

 <div id="container">
    <div id="left-col">
      <p>Test content</p>
      <p>longer</p>
    </div>
    <div id="right-col">
      <p>Test content</p>
    </div>
  </div>