is either a measurement of its containing block
  (if that is definite) or an infinite size (when it is indefinite).
  https://drafts.csswg.org/css-sizing-4/#available

  - - - - - - - - - - - - -

  Inside this batch of 24 sizing-orthog-htb-in-vlr tests:
  001-012 : body has 100px margin-left and 100px margin-right
  013-024 : body has no horizontal margins

  Inside each sub-batch of 12 tests:
  001-006: containing block's width is 'auto' therefore indefinite
  007-012: containing block's width is 400px t