The Web Developer add-on for Chrome tries to complement Chrome's already-excellent developer tools (Ctrl-Shift-I) with some in-page hints and tools. The garbled output you see above is the result of selecting Information > Display ID & Class Details. Not very graceful, obviously.
The add-on is missing a screen ruler (I'm sure the developer will add it later). Despite lacking a graceful way to show massive amounts of data, it can still come in handy every now and then. For example, you can disable CSS entirely, or just inline style, browser default styles, etc. That's pretty neat. It's still a fledgling add-on, so don't expect too much. But if you find Chrome's default tools are not enough for you, try giving it a shot.