The test started failing with text mismatch after http://trac.webkit.org/changeset/129070
Here is the link to the flakiness dashboard:
I'm going to update test expectations for now.
Still happening on Mac:
@@ -5,7 +5,7 @@
Testing value of 'disabled' prior to load just after setting them
PASS mainSheetLink.sheet is non-null.
-PASS alternateSheetLink.sheet is null
+FAIL alternateSheetLink.sheet should be null. Was [object CSSStyleSheet].
PASS mainSheetLink.disabled is true
PASS alternateSheetLink.disabled is false
Testing the values when the alternate sheet is loaded (as this is the only one that has sheet() === null)