The input is assumed to be a tree-of-boxes representation of a document where boxes are represented by css:box elements. The document root and top-level elements in named flows must be boxes or css:_ elements. The parent of a box must be another box (or a css:_ element if it's the document root or a top-level element in a named flow). Inline boxes must not have non-inline descendant boxes. Table-cell boxes must have a parent table box and table boxes must have only table-cell child boxes. All other nodes must have at least one inline box ancestor. If the input represents a named flow this must be indicated with a css:flow attribute on the document element.

Adjacent inline boxes with one or more sibling block or table boxes are grouped and wrapped in an anonymous block box. If all top-level boxes in the normal flow are inline they are wrapped in an anonymous block box too. Top-level elements in a named flow that are inline boxes are wrapped in an anonymous block box each and their css:anchor attributes are moved to the anonymous block. Top-level elements in a named flow that are css:_ elements are changed into anonymous block boxes.