This is a testharness.js-based test.
[FAIL] body marginwidth vs body leftmargin
  assert_equals: marginTop expected "20px" but got "10px"
[FAIL] iframe marginwidth vs child body leftmargin
  assert_equals: marginTop expected "20px" but got "10px"
[FAIL] iframe marginwidth vs child body marginwidth
  assert_equals: marginTop expected "20px" but got "10px"
Harness: the test ran to completion.