This is a combination of css debugging style discussed here: http://meyerweb.com/eric/thoughts/2007/09/07/diagnostic-styling/ http://www.nealgrosskopf.com/tech/thread.asp?pid=17 ... with additional tweaks and additions
The purpose of these styles are to highlight malformed, incorrect, incomplete, and wrongly nested HTML tags that would otherwise not be visible to the eye. Using these styles, HTML tags that match any of the above conditions will be highlighted with a thick dotted border with a color that reflects on the offense.
http://roykolak.github.io/css-debugger/ provides a page with a convenient bookmarklet that can be dragged to a bookmark bar to be triggered on any page.