Layout and the containing block
The size and position of an element are often impacted by its containing block. Most often, the containing block is the content area of an element's nearest block-level ancestor, but this is not always the case. In this article, we examine the factors that determine an element's containing block.
When a user agent (such as your browser) lays out a document, it generates a box for every element. Each box is divided into four areas:
- Content area
- Padding area
- Border area
- Margin area
Many developers believe that the containing block of an element is always the content area of its parent, but that isn't necessarily true. Let's investigate the factors that determine what an element's containing block is.
Effects of the containing block
Before learning what determines the containing block of an element, it's useful to know why it matters in the first place.
The size and position of an element are often impacted by its containing block. Percentage values that are applied to the width
, height
, padding
, margin
, and offset properties of an absolutely positioned element (i.e., which has its position
set to absolute
or fixed
) are computed from the element's containing block.
Identifying the containing block
The process for identifying the containing block depends entirely on the value of the element's position
property:
- If the
position
property isstatic
,relative
, orsticky
, the containing block is formed by the edge of the content box of the nearest ancestor element that is either a block container (such as an inline-block, block, or list-item element) or establishes a formatting context (such as a table container, flex container, grid container, or the block container itself). - If the
position
property isabsolute
, the containing block is formed by the edge of the padding box of the nearest ancestor element that has aposition
value other thanstatic
(fixed
,absolute
,relative
, orsticky
). - If the
position
property isfixed
, the containing block is established by the viewport (in the case of continuous media) or the page area (in the case of paged media). - If the
position
property isabsolute
orfixed
, the containing block may also be formed by the edge of the padding box of the nearest ancestor element that has the following:- A
transform
orperspective
value other thannone
- A
will-change
value oftransform
orperspective
- A
filter
value other thannone
or awill-change
value offilter
(only works on Firefox). - A
contain
value ofpaint
(e.g.contain: paint;
)
- A
Note: The containing block in which the root element (<html>
) resides is a rectangle called the initial containing block. It has the dimensions of the viewport (for continuous media) or the page area (for paged media).
Calculating percentage values from the containing block
As noted above, when certain properties are given a percentage value, the computed value depends on the element's containing block. The properties that work this way are box model properties and offset properties:
Some examples
The HTML code for all our examples is:
<body>
<section>
<p>This is a paragraph!</p>
</section>
</body>
Only the CSS is altered in each instance below.
Example 1
In this example, the paragraph is statically positioned, so its containing block is <section>
because it's the nearest ancestor that is a block container.
body {
background: beige;
}
section {
display: block;
width: 400px;
height: 160px;
background: lightgray;
}
p {
width: 50%; /* == 400px * .5 = 200px */
height: 25%; /* == 160px * .25 = 40px */
margin: 5%; /* == 400px * .05 = 20px */
padding: 5%; /* == 400px * .05 = 20px */
background: cyan;
}
Example 2
In this example, the paragraph's containing block is the <body>
element, because <section>
is not a block container (because of display: inline
) and doesn't establish a formatting context.
body {
background: beige;
}
section {
display: inline;
background: lightgray;
}
p {
width: 50%; /* == half the body's width */
height: 200px; /* Note: a percentage would be 0 */
background: cyan;
}
Example 3
In this example, the paragraph's containing block is <section>
because the latter's position
is absolute
. The paragraph's percentage values are affected by the padding
of its containing block, though if the containing block's box-sizing
value were border-box
this would not be the case.
body {
background: beige;
}
section {
position: absolute;
left: 30px;
top: 30px;
width: 400px;
height: 160px;
padding: 30px 20px;
background: lightgray;
}
p {
position: absolute;
width: 50%; /* == (400px + 20px + 20px) * .5 = 220px */
height: 25%; /* == (160px + 30px + 30px) * .25 = 55px */
margin: 5%; /* == (400px + 20px + 20px) * .05 = 22px */
padding: 5%; /* == (400px + 20px + 20px) * .05 = 22px */
background: cyan;
}
Example 4
In this example, the paragraph's position
is fixed
, so its containing block is the initial containing block (on screens, the viewport). Thus, the paragraph's dimensions change based on the size of the browser window.
body {
background: beige;
}
section {
width: 400px;
height: 480px;
margin: 30px;
padding: 15px;
background: lightgray;
}
p {
position: fixed;
width: 50%; /* == (50vw - (width of vertical scrollbar)) */
height: 50%; /* == (50vh - (height of horizontal scrollbar)) */
margin: 5%; /* == (5vw - (width of vertical scrollbar)) */
padding: 5%; /* == (5vw - (width of vertical scrollbar)) */
background: cyan;
}
Example 5
In this example, the paragraph's position
is absolute
, so its containing block is <section>
, which is the nearest ancestor with a transform
property that isn't none
.
body {
background: beige;
}
section {
transform: rotate(0deg);
width: 400px;
height: 160px;
background: lightgray;
}
p {
position: absolute;
left: 80px;
top: 30px;
width: 50%; /* == 200px */
height: 25%; /* == 40px */
margin: 5%; /* == 20px */
padding: 5%; /* == 20px */
background: cyan;
}
See also
- 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.
- The
all
property resets all CSS declarations to a given known state