This page shows the value of relative sizing. If you change the default font size on your browser, the relative font sizes below should change relatively. The font at 2em and 200% should be exactly twice the size of the default font.
This is the default font at the default size.
This is the default font at 1em.
This is the default font at 100%.
This is the default font at 1.2em.
This is the default font at 120%.
This is the default font at 1.5em.
This is the default font at 150%.
This is the default font at 2em.
This is the default font at 200%.