If Developer Tools are a complete secret to you, you may have had difficulty finding them!
As well as a menu shortcut, you can access the Developer Tools easily either pressing “F12” in Windows or “Cmd” ⌘, “Option” ⌥ and “I” on the Mac, or by right clicking on the page and selecting “Inspect Element”.
In Safari, you will need to enable the Developer Tools by checking the option in Preferences > Advanced > Show Develop Menu first.
In Opera, you will need to enable Developer Tools by checking the option in More Tools > Enable developer tools.
To see all errors on the page switch to Console tab in the Developer Tools and refresh page.