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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 one of the following:

  • the root element or something that contains it
  • floats (elements where float is not none)
  • absolutely positioned elements (elements where position is absolute or fixed)
  • 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)
  • anonymous table cells implicitly created by the elements with display: tabletable-rowtable-row-grouptable-header-group, table-footer-group (which is the default for HTML tables, table rows, table bodies, table headers and table footers, respectively), or inline-table 
  • block elements where overflow has a value other than visible
  • display: flow-root
  • elements with contain: layoutcontent, or strict
  • flex items (direct children of the element with display: flex or inline-flex)
  • grid items (direct children of the element with display: grid or inline-grid)
  • multicol containers (elements where column-count or column-width is not auto, including elements with column-count: 1)
  • column-span: all should always create a new formatting context, even when the column-span: all element 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.

Specifications

See also

Document Tags and Contributors

 Last updated by: SelenIT,