Better enforcement of spec-level rules via the type system
Properly represent text runs in the box tree as text runs rather than inline boxes
Boxes are no longer generated for the "document" or "doctype" nodes. Instead, the box tree is rooted from the HTML node.
Represent many more spec concepts — formatting contexts, outer-display and inner-display, block containers, anonymous {block, inline} boxes, the root-inline box...
Much cleaner scale factor implementation
Support for more writing-mode and direction combinations in layout
writing-modeanddirectioncombinations in layout