The big advantage of em over % for font size is you can use em to control width of other sections of the web site like line length and container divs.
An em is equal to the width of an uppercase M in that font face and point size, except on the web it is 16 pixels or the if the font size has been declare = to the font height.
I would suggest that it is to do with the width of an uppercase M in Times New Roman in the default size as the default font for a PC browser.
I did a little experimenting a couple of days ago with ems on my blog http://nickcowie.com/2005/about-em Nick This email is from the Department of Consumer and Employment Protection and any information or attachments to it may be confidential. If you are not the intended recipient, please reply mail to the sender informing them of the error and delete all copies from your computer system, including attachments and your reply email. As the information is confidential you must not disclose, copy or use it in any manner. ****************************************************** The discussion list for http://webstandardsgroup.org/ See http://webstandardsgroup.org/mail/guidelines.cfm for some hints on posting to the list & getting help ******************************************************
