Fixed font sizing, or absolute sizing, uses pixels or inches. Fixed font sizing is not the recommended way to establish font-size values because unlike relative sizing the relationship between the text does not remain proportional when fonts are scaled up or scaled down by simply adjusting the font-size property of the body selector. Only the the text controlled by the body tag changes. All other text remains at its original size.
This paragraph's font-size property is 12px and is set in the body selector to 12px, which is equivalent to 1em set at 75%.