Block formatting context

A block formatting context is a part of a visual CSS rendering of a web page. It's 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 of the document (<html>).
  • Floats (elements where float isn't 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: table, table-row, table-row-group, table-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 and clip.
  • display: flow-root.
  • Elements with contain: layout, content, or paint.
  • Flex items (direct children of the element with display: flex or inline-flex) if they are neither flex nor grid nor table containers themselves.
  • Grid items (direct children of the element with display: grid or inline-grid) if they are neither flex nor grid nor table containers themselves.
  • Multicol containers (elements where column-count or column-width isn't 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).

Formatting contexts affect layout, but typically, we create a new block formatting context for the positioning and clearing floats rather than changing the layout, because an element that establishes a new block formatting context will:

Note: a Flex/Grid container(display: flex/grid/inline-flex/inline-grid) establishes a new Flex/Grid formatting context, which is similar to block formatting context except layout. There's no floating children avaliable inside a flex/grid container, but exclude external floats and suppress margin collapsing still works.

Examples

Contain internal floats

Make float content and alongside content the same height.

Let's have a look at a couple of these in order to see the effect creating a new BFC.

In the following example, 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.

using overflow: auto

Setting overflow: auto or set other values than the initial value of overflow: visible 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 want 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 readable for a future developer, as it might not be obvious why you used overflow for this purpose. If you use overflow, it is a good idea to comment the code to explain.

using display: flow-root

A newer value of display lets us create a new BFC without any other potentially problematic side-effects. Using display: flow-root on the containing block creates a new BFC .

With 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> element in browser) in terms of how it creates a new context for the flow layout inside it.

HTML

<section>
    <div class="box">
        <div class="float">I am a floated box!</div>
        <p>I am content inside the container.</p>
    </div>
</section>
<section>
    <div class="box" style="overflow:auto">
        <div class="float">I am a floated box!</div>
        <p>I am content inside the <code>overflow:auto</code> container.</p>
    </div>
</section>
<section>
    <div class="box" style="display:flow-root">
        <div class="float">I am a floated box!</div>
        <p>I am content inside the <code>display:flow-root</code> container.</p>
    </div>
</section>

CSS

section {
    height:150px;
}
.box {
    background-color: rgb(224, 206, 247);
    border: 5px solid rebeccapurple;
}
.box[style] {
    background-color: aliceblue;
    border: 5px solid steelblue;  
}
.float {
    float: left;
    width: 200px;
    height: 100px;
    background-color: rgba(255, 255, 255, .5);
    border:1px solid black;
    padding: 10px;
}

Exclude external floats

In the following example, we are using display:flow-root and floats to implement double columns layout, because an element in the normal flow that establishes a new BFC must not overlap the margin box of any floats in the same block formatting context as the element itself.

HTML

<section>
  <div class="float">Try to resize this outer float</div>
  <div class="box"><p>Normal</p></div>
</section>
<section>
  <div class="float">Try to resize this outer float</div>
  <div class="box" style="display:flow-root"><p><code>display:flow-root</code><p></div>
</section>

CSS

section {
    height:150px;
}
.box {   
    background-color: rgb(224, 206, 247);   
    border: 5px solid rebeccapurple; 
} 
.box[style] {   
    background-color: aliceblue;   
    border: 5px solid steelblue;   
} 
.float {   
    float: left;
    overflow: hidden; /* required by resize:both */
    resize: both;
    margin-right:25px;
    width: 200px;   
    height: 100px;   
    background-color: rgba(255, 255, 255, .75);   
    border: 1px solid black;   
    padding: 10px; 
}

Rather than inline-blocks with width:<percentage>, in this case we don't have to specify the width of the right div.

Note that flexbox is a more efficient way to implement muti columns layout in morden CSS.

Margin collapsing

Creating a new BFC to avoid the margin collapsing between two neighbor div:

HTML

<div class="blue"></div>
<div class="red-outer">
  <div class="red-inner">red inner</div>
</div>

CSS

.blue, .red-inner {
  height: 50px;
  margin: 10px 0;
}

.blue {
  background: blue;
}

.red-outer {
  overflow: hidden;
  background: red;
}

Specifications

Specification Status Comment
CSS Display Module Level 3
The definition of 'Block Formatting Context' in that specification.
Candidate Recommendation define BFC(abbr) etc.
CSS Level 2 (Revision 1)
The definition of 'Block Formatting Context' in that specification.
Recommendation Initial definition.

See also