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 other elements.
A block formatting context is created by at least one of the following:
- the root element or something that contains it
- floats (elements where
- absolutely positioned elements (elements where
- inline-blocks (elements with
- table cells (elements with
: table-cell, which is the default for HTML table cells)
- table captions (elements with
: table-caption, which is the default for HTML table captions)
- anonymous table cells implicitly created by the elements with
table-footer-group(which is the default for HTML tables, table rows, table bodies, table headers and table footers, respectively), or
- block elements where
overflowhas a value other than
- elements with
- flex items (direct children of the element with
- grid items (direct children of the element with
- multicol containers (elements where
auto, including elements with
: allshould always create a new formatting context, even when the
column-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.
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 the content inside other block formatting contexts, and clear only clears past floats in the same block formatting context. Margin collapsing also occurs only between blocks that belong to the same block formatting context.
- CSS Key Concepts: CSS syntax, at-rule, comments, specificity and inheritance, the box, layout modes and visual formatting models, and margin collapsing, or the initial, computed, resolved, specified, used, and actual values. Definitions of value syntax, shorthand properties and replaced elements.