Are there any practical reasons to use “em” instead of “pt” font size units?

Question

One CSS rule I have learned is that you should use the relative \"em\" font-size unit instead of the absolute \"pt\". The general idea is to set the font-size in your body t

Answer 1

Even if you hard-code the font-size in pixel, you can still use em for unit to specify margin, length, etc.. , similar to using em quad to indent a paragraph in the printing press anyway.

Edit (After the poster changed from px to pt): If you want to be "pixel perfect", it's safer to go with px rather than pt, since different operating system has different dpi setting, and user change change dpi dramatically especially on Linux. PostScript point is defined to be 1/72 of an inch. An "inch" on screen can be anywhere between 72 pixels to whatever floats your boat.