iOS 6 brings a new debugging feature.
For displaying layers used for rendering a UIWebView's content, set the WebKitShowDebugBorders to YES and restart the application.
[[NSUserDefaults standardDefaults] setBool: YES forKey:@"WebKitShowDebugBorders"];
The layers will be drawn with a border with one of the colors below:
Container
Clipped
Backing Store
Tiled