2. Em is a typeface measurement that was originally defined as a measurement between the top and bottom of the capital M. However, today an em is a dynamic measurement based off the point size value of the font-size property. For example, in CSS if you set the font-size of H1 to 1.2em its text size would be 20% greater than the font-size it inherits from the parent element or the default font property. See our em space definition for further information and examples.
3. Chat slang for them.