If a picture speaks a thousand words then a mockup speaks a thousand lines of code. Pixel perfection is the only standard operating procedure worth mentioning ever since WYSIWYG came into vogue in the 90's. For conceptual drafting of web page progression, nothing beats spending some time in good old Photoshop to nail an overall look and a site's particular elements. Some designers use slicing to cut up a mockup and make it a site. In my mind, that's been outdated since the advent of CSS. If you can't code what you can mock, then learn more.