The W3C Cascading Stylesheets Working Group has published working drafts of CSS Advanced Layout Module and CSS basic box model. According to the latter,
When textual documents (e.g., HTML) are laid out on visual media (e.g., screen or print), CSS models the document as a hierarchy of boxes containing words, lines, paragraphs, tables, etc. each with properties such as size, color and font.
This module describes the basic types of boxes, with their padding and margin, and the normal “flow” (i.e., the sequence of blocks of text with margins in-between). It also defines “floating” boxes, but other kinds of layout, such as tables, absolute positioning, ruby annotations, grid layouts, columns and numbered pages, are described by other modules. Also, the layout of text inside each line (including the handling of left-to-right and right-to-left scripts) is defined elsewhere.
Boxes may contain either horizontal or vertical lines of text. Boxes of different orientations may be mixed in one flow. (This is a level 3 feature.)
The advanced layout module:
contains features to describe layouts at a high level, meant for tasks such as the positioning and alignment of “widgets” in a graphical user interface or the layout grid for a page or a window, in particular when the desired visual order is different from the order of the elements in the source document. Other CSS3 modules contain properties to specify fonts, colors, text alignment, list numbering, tables, etc.
