If you only want one guideline, make this it. Because, frankly, for all the reasons already given, there really is nothing more irritating to readers than design for design's sake. This isn't to say your web page shouldn't use all the design elements and special touches that create style, pace, flair, excitement, intrigue, emotion. Of course it should! But these elements must always be relevant and appropriate, and not distract from a clear, coherent whole effect. Remember this, and you'll vastly increase your chances of creating effective online design - a design that draws attention to the message, not to itself - a design that will serve your web site visitors rather than dazzle them.