1. Open this page in Internet Explorer.
  2. Go to View > Text Size and resize the text size

You'll see that the bottom two lines that are set to resize but not the top line that is set to Pixles.

This line is set to 16px. (This is the default font size of webpages).

This line is set to 1 em. (1em = 16px)

This line is set to 100%. (100% = 16px)