Web Animations API: Avoid style resolution when calling element.animate()

LayoutTests/animations/interpolation/font-size-interpolation.html

Index: LayoutTests/animations/interpolation/font-size-interpolation.html
diff --git a/LayoutTests/animations/interpolation/font-size-interpolation.html b/LayoutTests/animations/interpolation/font-size-interpolation.html
index 785e428841f83a2212d2f405d1385d0e30b4ecdb..833cce5d576280700bb06146eee3aeb13e11c26c 100644
--- a/LayoutTests/animations/interpolation/font-size-interpolation.html
+++ b/LayoutTests/animations/interpolation/font-size-interpolation.html
@@ -27,5 +27,18 @@ assertInterpolation({
   {at: 1, is: '14px'},
   {at: 1.5, is: '19px'},
 ]);
+assertInterpolation({
+  property: 'font-size',
+  from: 'x-small',
+  to: '20px'
+}, [
+  {at: -2, is: '0px'},  // CSS font-size can't be negative.
+  {at: -0.3, is: '7px'},
+  {at: 0, is: '10px'},
+  {at: 0.3, is: '13px'},
+  {at: 0.6, is: '16px'},
+  {at: 1, is: '20px'},
+  {at: 1.5, is: '25px'},
+]);
 </script>
 </body>