Footnotes:

1 For layout, at least. It might be revisited
  for painting, for building the accessibility tree if necessary,
  and for hit testing if necessary.

2 Reality, of course, is a bit more
  complicated. Some layouts involve intrinsic dimensions or baseline
  measurements, which do involve an additional walk of the relevant subtree
  (aggressive caching is used to mitigate the potential for quadratic
  performance in the worst case). These cases, however, are surprisingly
  rare. In particular, intrinsic dimensions are not required for the
  common case of shrink-wrapping.