I have yet to root-cause this bug, but it is the reason that we fail the following tests:

external/wpt/web-animations/animation-model/animation-types/discrete-animation.html
  - Test the 50% switch point for discrete animation is based on the effect easing
  - Test the 50% switch point for discrete animation is based on the keyframe easing 

A simpler test shows that Chrome does not appear to support animating the fontStyle at all:

  var div = document.getElementById("testDiv");

  var anim = div.animate({ fontStyle: [ 'italic', 'oblique' ] },
                         { duration: 1000, fill: 'forwards' });
  
  for (var time = 0; time <= 1000; time += 100) {
    anim.currentTime = time;
    console.log("time " + time + ", getComputedStyle(div).fontStyle: " + getComputedStyle(div).fontStyle);
  }

In Firefox this produces:

time 0, getComputedStyle(div).fontStyle: italic
time 100, getComputedStyle(div).fontStyle: italic
time 200, getComputedStyle(div).fontStyle: italic
time 300, getComputedStyle(div).fontStyle: italic
time 400, getComputedStyle(div).fontStyle: italic
time 500, getComputedStyle(div).fontStyle: oblique
time 600, getComputedStyle(div).fontStyle: oblique
time 700, getComputedStyle(div).fontStyle: oblique
time 800, getComputedStyle(div).fontStyle: oblique
time 900, getComputedStyle(div).fontStyle: oblique
time 1000, getComputedStyle(div).fontStyle: oblique

In Chrome this produces:

time 0, getComputedStyle(div).fontStyle: italic"
time 100, getComputedStyle(div).fontStyle: italic
time 200, getComputedStyle(div).fontStyle: italic
time 300, getComputedStyle(div).fontStyle: italic
time 400, getComputedStyle(div).fontStyle: italic
time 500, getComputedStyle(div).fontStyle: italic
time 600, getComputedStyle(div).fontStyle: italic
time 700, getComputedStyle(div).fontStyle: italic
time 800, getComputedStyle(div).fontStyle: italic
time 900, getComputedStyle(div).fontStyle: italic
time 1000, getComputedStyle(div).fontStyle: italic


As far as the spec says, all properties are animatable so this should work (https://w3c.github.io/web-animations/#procedures-for-animating-properties):

"Unless specifically defined otherwise, all CSS properties are considered animatable."