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:
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.
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.
The process for identifying the containing block depends entirely on the value of the element's position
property:
position
property is static
orrelative
, the containing block is formed by the edge of the content box of the nearest ancestor element that is a block container (such as an inline-block, block, or list-item element) or which establishes a formatting context (such as a table container, flex container, grid container, or the block container itself).position
property is absolute
, the containing block is formed by the edge of the padding box of the nearest ancestor element that has a position
value other than static
(fixed
, absolute
, relative
, or sticky
).position
property is fixed
, the containing block is established by the viewport (in the case of continuous media) or the page area (in the case of paged media).position
property is absolute
or fixed
, the containing block may also be formed by the edge of the padding box of the nearest ancestor element that has the following: transform
or perspective
value other than none
will-change
value of transform
or perspective
filter
value other than none
or a will-change
value of filter
(only works on Firefox).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).
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:
height
, top
, and bottom
properties compute percentage values from the height
of the containing block. If the heightof the containing block depends on its contents, these values become 0
when the containing block has a position
of relative
or static
.width
, left
, right
, padding
, and margin
properties compute percentage values from the width
of the containing block.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.
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; }
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; }
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; }
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; }
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; }
all
property resets all CSS declarations to a given known state
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https://developer.mozilla.org/en-US/docs/Web/CSS/Containing_block