XRAY is a little browser bookmarklet that displays all the properties associated with the selected element and “box model for any element on any web page”. XRAY shows you the sizes and other attributes of element you select, fades the rest of the page out and hovers a panel over the page to show the properties.
The X-Ray extension for Firefox will let you see HTML tags without looking at the source code. Once installed the X-Ray command is available by right-clicking as well as in the Tools menu. It helps you to see how the document was constructed without having to go back and forth between the sourcecode and the page in your browser. X-Ray shows you what’s beneath the surface of the page.