Intermittent failure in /css/css-backgrounds/background-size-027.html

```
  ▶ FAIL [expected PASS] /css/css-backgrounds/background-size-027.html
  └   → /css/css-backgrounds/background-size-027.html 10b5c8141f3140455d638499eb70869bb196564c
/css/css-backgrounds/reference/background-size-027-ref.xht 0024c206b1bc33e4ac813ebc89459ae9e43ff5cc
Testing 10b5c8141f3140455d638499eb70869bb196564c == 0024c206b1bc33e4ac813ebc89459ae9e43ff5cc
```