processTree static method
processTree optimizes the StyledElement tree so all BlockElements are
on the first level, redundant levels are collapsed, empty elements are
removed, and specialty elements are processed.
Implementation
static StyledElement processTree(
    StyledElement tree, double devicePixelRatio) {
  tree = _processInternalWhitespace(tree);
  tree = _processInlineWhitespace(tree);
  tree = _removeEmptyElements(tree);
  tree = _calculateRelativeValues(tree, devicePixelRatio);
  tree = _preprocessListMarkers(tree);
  tree = _processCounters(tree);
  tree = _processListMarkers(tree);
  tree = _processBeforesAndAfters(tree);
  tree = _collapseMargins(tree);
  return tree;
}