Web Form Design: Filling in the Blanks by Luke Wroblewski Rosenfeld Media, 2008; version 1.0
Many people will answer “It means this input is required so I have to provide an answer.” Many—but not all. As is common with all Web conventions (de facto standards that have organically emerged online), there are exceptions to this rule. In fact, some Web sites use an asterisk to indicate optional input fields (Figure 5.5) instead of required ones.
Figure 5.5 http://www.flickr.com/photos/rosenfeldmedia/2367266030 Asterisks on the Hogan sign-up form indicate optional fields! Not what most people would expect.
120 Chapter 5 Input Fields Download at Boykma.Com