Intermittent results in /css/css-backgrounds/background-size-025.html

```
  ▶ FAIL [expected PASS] /css/css-backgrounds/background-size-025.html
  └   → /css/css-backgrounds/background-size-025.html 2e086078beb87d833c6d8bba01b10fd0470bc2d7
/css/css-backgrounds/reference/background-size-025-ref.xht 1f38014d88e0025ae8ccfc55209367830ccd147a
Testing 2e086078beb87d833c6d8bba01b10fd0470bc2d7 == 1f38014d88e0025ae8ccfc55209367830ccd147a
```