This isn't documented anywhere that I know of nor is it in the spec, but you if you add a results parameter on the input, WebKit will apply a little magnifying glass with a dropdown arrow showing previous results.
For a font size of 16px, the perfect line height is achieved when h equals the golden ratio. This yields a value of 25.88854px for the optimal line height. Using this value, you can then determine that the associated optimal line width is 670.21670px.