There are quite a few data types for property values. font-size, for example, accepts keywords, lengths and percentages.

<html>
 <head>
  <style>p {font-size: 1.125em;}</style>
 </head>
 <body><p>Dream of para-para-paragraphs</p></body>
</html>
]]>

See that? If we use data types that are based on numbers, we can use whole or floating-point numbers.

That doesn't look right, does it?