Block of content

A block of content in a web page is a set of nodes from that page such that:

(content) there is at least one node of perceivable content in the block; and
(continuity) if two nodes are in the block, then any node between them (in tree order) is also in the block; and
(downward closure) if a node is in the block, then all its descendants are also in the block; and
(parent closure) if all children of a node are in the block, then this node is also in the block.

When the rendering order of nodes on a page greatly differs from the DOM tree order, blocks of content as defined here may be different from what is visually perceived as "content in a close relationship".