Most all web developers, most certainly those who work with Firefox, are familiar with Firebug. This tools nevers ceases to amaze me, the ability to change information (for example style attributes) on a webpage as your are viewing the page, is amazingly useful. Version 3.x of Safari now includes the option to enable a Firebug like debugging tool called Web Inspector
You wont find access to Web Inspector in your default installation of Safari. However, making the necessary configuration changes is as easy as running the following command from a Terminal:
defaults write com.apple.Safari WebKitDeveloperExtras -bool true
If you prefer, you can use the Property List Editor to set the WebKitDeveloperExtras property in the Safari property file, which is located here: ~/Library/Preferences/com.apple.Safai.plist (use Finder to locate the file and double click to open). Youll need to restart Safari after making the changes to enable Web Inspector.
To open the Web Inspector, right click and choose Inspect Element as shown in the figure below:
With Web Inspector running, you have access to a goodly amount of information. Take a look at the screenshot below which shows content from the Google homepage.
While poking around on Web Inspector, I clicked on icon in the lower left corner (the little arrow-in-a-box) and a popup menu appeared showing options for Network and Console. I selected Network and information about document and image transfers appeared (see the figure below). The Console option in this same menu resulted in a blank page appearing, so it seems I still have some exploring to do to figure what this option is all about. If you click on the icon just to the right of the arrow-in-a-box, the Web Inspector will be shown in a split window view versus a popup window.
I havent found an option for editing on-the-fly as in Firebug, however, Web Inspector is a good start for a built in tool and I have no doubt it will continue to evolve.