This looks like a nice way to analyze the DOM structure of websites:
The graph above shows the analysis of http://www.photomeister.com
The colors used for the nodes are:
blue: for links (the A tag)
red: for tables (TABLE, TR and TD tags)
green: for the DIV tag
violet: for images (the IMG tag)
yellow: for forms (FORM, INPUT, TEXTAREA, SELECT and OPTION tags)
orange: for linebreaks and blockquotes (BR, P, and BLOCKQUOTE tags)
black: the HTML tag, the root node
gray: all other tags
It comes to no surprise that google.com has a very clean DOM structure:
The developer of the tool has more samples on his website.
This link takes you directly to the tool.