Summary
A block formatting context is a part of a visual CSS rendering of a Web page. It is the region in which the layout of block boxes occurs and in which floats interact with each other.
A block formatting context is created by one of the following:
- the root element or something that contains it
- floats (elements where
floatis notnone) - absolutely positioned elements (elements where
positionisabsoluteorfixed) - inline-blocks (elements with
display: inline-block) - table cells (elements with
display: table-cell, which is the default for HTML table cells) - table captions (elements with
display: table-caption, which is the default for HTML table captions) - block elements where
overflowhas a value other thanvisible display: flow-rootcolumn-span: allshould always create a new formatting context, even when thecolumn-span: allelement isn't contained by a multicol container(Spec change, Chrome bug).
A block formatting context contains everything inside of the element creating it that is not also inside a descendant element that creates a new block formatting context.
Block formatting contexts are important for the positioning (see float) and clearing (see clear) of floats. The rules for positioning and clearing of floats apply only to things within the same block formatting context. Floats do not affect the layout of things in other block formatting contexts, and clear only clears past floats in the same block formatting context.