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Pro Measures visible not HTML elements
Dimensions takes a screenshot of the page and uses that to create a contour map via edge detection that can then be used to measure actual page elements instead of HTML elements. For example, Dimensions measures against contours of letters in text, through transparent backgrounds, etc
Con Doesn't work well with gradients and textured backgrounds
Since Dimensions is based on color edge detection, color gradients and patterns tend to mess with the results. A simple workaround is to open Chrome's DevTools (or equivalent) and remove the offending DOM object.