Can CSS3 transition font size?

The color transitions fine over time, but the font switches
immediately for some dagnabbit reason.

Your font-size transition is being overwritten by your color transition.

transition: font-size 12s; /* transition is set to 'font-size 12s' */
transition: color 12s;     /* transition is set to 'color 12s' !! */

Instead, you must combine them all into one declaration:

transition: color 12s, font-size 12s;

See: http://jsfiddle.net/thirtydot/6HCRs/

-webkit-transition: color 12s, font-size 12s;
   -moz-transition: color 12s, font-size 12s;
     -o-transition: color 12s, font-size 12s;
        transition: color 12s, font-size 12s;

(Or, just use the all keyword: transition: all 12s; – http://jsfiddle.net/thirtydot/6HCRs/1/).