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.
Let's have a look at a couple of these in order to see the effect creating a new BFC.
In the example below, we have a floated element inside a
<div> with a border applied. The content of that
div has floated alongside the floated element. As the content of the float is taller than the content alongside it, the border of the div now runs through the float. As explained in the guide to in-flow and out of flow elements, the float has been taken out of flow so the background and border of the div only contain the content and not the float.
Creating a new BFC would contain the float. A typical way to do this in the past has been to set
overflow: auto or set other values than the initial value of
overflow: auto created a new BFC containing the float. Our
div now becomes a mini-layout inside our layout. Any child element will be contained inside it.
The problem with using
overflow to create a new BFC is that the
overflow property is meant for telling the browser how you wish to deal with overflowing content. There are some occasions in which you will find you get unwanted scrollbars or clipped shadows when you use this property purely to create a BFC. In addition, it is potentially not very readable for a future developer, as it may not be obvious why you used overflow for this purpose. If you do this, it would be a good idea to comment the code to explain.
A newer value of
display gives us the ability to create a new BFC without any other potentially problematic side-effects. Using
display: flow-root on the containing block will create a new BFC .
display: flow-root; on the
div, everything inside that container participates in the block formatting context of that container, and floats will not poke out of the bottom of the element.
The value name of
flow-root makes sense when you understand you are creating something that acts like the
root element (
html) in terms of how it creates a new context for the flow layout inside it.
- 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.