HTML - What is the pixel width of an Em?

1) The em is the font-size of the current font.

It is a relative measure, so:
1 em equals 1x the height of the current font.
2 em equals 2x the height of the current font.

2) The point is a unit of measure from printed type, and is equal to ¹⁄₇₂ of an inch.

Which means the height of a 12 pt font is: 12 * ¹⁄₇₂ inch = ¹⁄₆ inch.

3) To determine the font height on screen, versus the printed page, we need to convert ¹⁄₆ inch to pixels.

The CSS specification defines a pixel to be ¹⁄₉₆ inch.

And ¹⁄₆ inch * 96 pixels per inch = 16 px.