1. A computer-implemented method comprising: receiving a structured document; encountering an opening tag within the structured document; creating a box builder associated with the opening tag, the box builder being one of multiple different types of box builders, each of the multiple different types of box builders specifically configured to build a particular type of content, and each of the multiple different types of box builders comprising an abstract class that represents common properties and methods of each specific one of the multiple different types of box builders, the box builder being further configured to be interrupted at a first point in building the particular type of content and restarted from a second point that is different than the first point, the second point being different from a beginning of the structured document and different from a third point that is sequential to the first point; utilizing a layout node that is configured to unify both original content and computed content, the computed content comprising values or data dependent on content properties and not dependent on space characteristics, the original content being dependent at least on space characteristics; creating an associated layout box that is re-usable in different positions for subsequent and parallel layout building processes based on the associated layout box containing internal positioned content and sizing information and not containing positioning data of the layout box; and using the box builder to sequentially process the structured document and populate the layout box with renderable content.