So-called ems are relative length units that depend on the default font size. For example, 0.5em stands for half the default font size.

<html>
 <head>
  <style>p {font-family: "Lucida Grande";font-size: 0.5em;}
  </style>
 </head>
 <body><p>Dream of para-para-paragraphs</p></body>
</html>
]]>

Good job! Ems are great on smartphone screens because they leave the question of exactly how big to make something to the web browser.

Didn't we want to go for a relative length unit?