don't tell someone which uses firefox to use internet explorer "because everybody else does it"
don't think of designing pixel wise, don't expect people to have your screen configuration so they don't see scrollbars when you don't see them
don't automatically resize the window of the site to fit the maximum width and height - your 1024 x 768px which seem absolutely great may be absolutely stupid if the user has 2048 or 1600 pixels wide screen
don't use <i> and <b> - they are deprecated and you don't want to look like obsolete. use <em> and <strong> instead
don't use <br>, and <img src="spacer.gif"> to position elements - that's designing pixel wise and absolutely demode...
don't even pronounce the words "optimized for..."
don't open pop-up's to view a site