by Ian Haynes.

One of the new CSS3 constructs that gets less attention than it deserves is that of ‘root em’ font sizing, normally know as ‘rem’. This makes all font sizes relative to the root <html> element of a page, not relative to the parent element. At a stroke it eliminates all the cascading effects which are the basis of CSS but can be a pain when sizing fonts in different areas of a page.

This article by Jonathan Snook explains it well:

One caveat in using rem is that the design surfaces in Visual Studio 2010/2011 and Expression Web don’t yet support it. The browser fallback method Jonathan suggests solves this partially though and lets us see the relative font sizes when working with these tools.

Microsoft MVP for Expression Web