Issue #17321. Under Linux, using "font-family: sans-serif" previously
caused Servo to select the "UltraLight" face (of DejaVu Sans). There
were two reasons for this:

1. Font weight was only retrieved from the OS/2 table for bold faces.
   This neglected to retrieve the weight information for "lighter than
   normal" weight faces. This meant that the UltraLight face appeared as
   normal weight, and was selected.

2. Retrieval of font stretch information from the OS/2 table was not
   implemented at all.