A physical EM ruler was requested as a joke on Twitter [1], but I believe a relative unit screen ruler would be both achievable and useful in the Web Inspector. PX and REM units would be very useful and easier to implement as a 1.0 version of the enhancement. EM and % would also be possible, but the UI may be difficult to get right; would you use the relative unit of the element under the mouse? Selected in the inspector? Both? Displaying EMs and % may be especially confusing for inexperienced CSS authors that don't understand they are multipliers of the ancestor nodes' font sizes. In any case, PX and REM seem like a good start. 1. https://twitter.com/harryharrold/status/449556995277074432
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