Inspector Ready to Use
defaults write com.apple.Safari WebKitDeveloperExtras -bool true
Lucky for developers, with single line of command as shown above run from Terminal, Web Inspector that can scan through web pages, like Firebug in Firefox, will be ready to use.
Many things you can inspect with this tool, from the most common stylesheets up to the time used by Network to retrieve the data. As you can see on image below.
Inspect Wherever You Want
Placing mouse over a certain portion of the design followed by triggering Web Inspector, that can be done by the assistance of contextual menu, will result in the Safari Web Inspector spotlighting and showing the details.
To my disappointment, the spotlight position is relative to the browser window which means that when you scroll down the page, the spotlight will follow you; This ruins the good impression I initially got when realizing its existence.
Nightly Builds, which contain many kinds of WebKit Inspector, can be downloaded to enhance your experience with Inspector.
While waiting for the download to complete, you can watch this short clip that gives a brief explanation to web inspector.
Disable Web Inspector
defaults write com.apple.Safari WebKitDeveloperExtras -bool false