The brightness of the lines is determined by the importance of the tags in terms of structure.

Basically a semantically rich site will appear brighter than one with messy old-style code.

You can also determine the richness of text on a site. A site the focuses on (text) content is one where the DNA patterns is large (filling many containers), but contains a lot of empty spaces between the lines (empty space is the individual words).

try it, via Infosthetics, Another tag-structure visualization