nes below, the resulting size is
        based the "base size" of the generic font (or the default font size if
        there is no generic font). Right now in Servo that size for serif is 16px
        by default -->
<p style="font-size: xx-small">xx-small is 9px.
<p style="font-size: x-small">x-small is 10px.
<p style="font-size: small">small is 13px.
<p style="font-size: medium">medium is 16px.
<p style="font-size: large">large is 18px.
<p style="font-size: x-large">x-large is 24px.
<p style="font-size: x