design: hierarchy / text / lightness values

Since a website usually has very little white space (space which is empty of informational content, that is) the hierarchical components of a site will often end up being placed very close to one another. 

The two images above should hopefully explain it: If you make all of your text one color (white in this case), it will be quite difficult for people to know where to look first when they come to your site. If however, you use various tones of gray then it will become far more obvious where the most important stuff is, and then what comes after that. As a rule, things on the side bars are less important than your post, these should be darker than the post. Inside those, visited links are less important than unvisited ones, and the rollover link should pop out the most.

Inside the post itself it is a good idea to highlight or make bold the stuff which you would like to stand out. (I prefer highlight since I do not like the way bold text looks on websites at small sizes - becomes smudgy).

Size is also an element that can be used for establishing visual hierarchies, however since most of the text will have to be on the small side you can only really use that for post titles and the header: When it comes to body text, a change from 13 pixels to 14 or 15 is not going to give you enough difference and will only make your design look 'ambiguous' - a term which designers fear like death! So, tonal values for type hierarchies are one of the best ways to go in web design, I think.