What happens if a Cascading Style Sheet (CSS) feature is used in your Web page code, but it isn't supported by a user's browser or OS? Nothing happens. Unlike programming languages -- with their often baffling error messages that can scare users -- browsers are designed to hide problems from their users. The user never sees an error message saying: "This CSS style is unsupported by Netscape." Instead, whatever special effect you were trying to achieve by redefining an HTML element with CSS is simply ignored. If you had redefined <h1> as a blue headline, that redefinition is ignored and the default black is just used instead. If CSS is controlling positioning, however, the results can be less benign. But, again, the damage is limited to those few people not using Internet Explorer.