One can read markup like music and gain an appreciation for a document as a complete product—but when the presentation is completely removed into its own dedicated scope, the only way to make sense of the result is to ride two trains of thought in parallel. For most people, that kind of perfectly balanced multitasking takes practice.
However, there are tools and habits that make it easier. Among the tools are the developer interfaces provided with every major browser, all designed to display related markup and stylesheet rules in context.
Habits are a different matter. Remember that the Web is a multidimensional space: if you see markup or CSS and your thought process is still constrained to only two (or even three) dimensions, you’re not seeing the whole picture.
Most of this section deals with presentation-layer development practice on a theoretical level. If you want to skip to the “climax” of this material, you should read Taxonomy and Nomenclature.
The Web has no beginning and no end. Individual pages might be bounded, but even those usually contain links to other sources of information, and visitors could have arrived from any context. Even when a document ends, the user experience doesn’t.
Working in this unbounded environment requires a tight focus on information in preference to any notion of the finished site. Everything else takes lower priority: designers’ hangups, branding guidelines, even ...