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The width CSS property specifies the width of an element. By default, the property defines the width of the content area. If box-sizing is set to border-box, however, it instead determines the width of the border area.

/* <length> values */
width: 300px;
width: 25em;

/* <percentage> value */
width: 75%;

/* Keyword values */
width: 25em border-box;
width: 75% content-box;
width: max-content;
width: min-content;
width: available;
width: fit-content;
width: auto;

/* Global values */
width: inherit;
width: initial;
width: unset;

注意: min-widthmax-width 會覆寫 width.

預設值auto
Applies toall elements but non-replaced inline elements, table rows, and row groups
繼承與否no
Percentagesrefer to the width of the containing block
Mediavisual
Computed valuea percentage or auto or the absolute length
Animation typea length, percentage or calc();
Canonical orderthe length or percentage before the keyword, if both are present

Syntax

The width property is specified as either:

Values

<length>
Defines the width as an absolute value.
<percentage>
Defines the width as a percentage of the containing block's width. If the width of the containing block depends on the width of the element, the resulting layout is undefined.
border-box
If present, the preceding <length> or <percentage> is applied to the element's border box.
content-box
If present, the preceding <length> or <percentage> is applied to the element's content box.
auto
The browser will calculate and select a width for the specified element.
fill
Use the fill-available inline size or fill-available block size, as appropriate to the writing mode.
max-content
The intrinsic preferred width.
min-content
The intrinsic minimum width.
available
The containing block width minus horizontal margin, border and padding.
fit-content
The larger of:
  • the intrinsic minimum width
  • the smaller of the intrinsic preferred width and the available width

Formal syntax

[ <length> | <percentage> ] && [ border-box | content-box ]? | available | min-content | max-content | fit-content | auto

範例

預設寬度

p.goldie {
  background: gold;
}
<p class="goldie">The Mozilla community produces a lot of great software.</p>

像素和相對大小

.px_length {
  width: 200px;
  background-color: red;
  color: white;
  border: 1px solid black;
}

.em_length {
  width: 20em;
  background-color: white;
  color: red;
  border: 1px solid black;
}
<div class="px_length">Width measured in px</div>
<div class="em_length">Width measured in em</div>

百分比

.percent {
  width: 20%;
  background-color: silver;
  border: 1px solid red;
}
<div class="percent">Width in percentage</div>

內容最大值

p.maxgreen {
  background: lightgreen;
  width: intrinsic;           /* Safari/WebKit uses a non-standard name */
  width: -moz-max-content;    /* Firefox/Gecko */
  width: -webkit-max-content; /* Chrome */
}
<p class="maxgreen">The Mozilla community produces a lot of great software.</p>

內容最小值

p.minblue {
  background: lightblue;
  width: -moz-min-content;    /* Firefox */
  width: -webkit-min-content; /* Chrome */
}
<p class="minblue">The Mozilla community produces a lot of great software.</p>

規範

Specification Status Comment
CSS Basic Box Model
The definition of 'width' in that specification.
Working Draft Added the max-content, min-content, available, fit-content, border-box, content-box keywords.
CSS Transitions
The definition of 'width' in that specification.
Working Draft Lists width as animatable.
CSS Level 2 (Revision 1)
The definition of 'width' in that specification.
Recommendation Precises on which element it applies to.
CSS Level 1
The definition of 'width' in that specification.
Recommendation Initial definition.
CSS Intrinsic & Extrinsic Sizing Module Level 3
The definition of 'width' in that specification.
Working Draft Adds new sizing keywords for width and height.

瀏覽器相容性

FeatureChromeEdgeFirefoxInternet ExplorerOperaSafari
Basic support Yes Yes143.51
Animatable ? ?16 ? ? ?
max-content

46

22 -webkit-

No3 -moz- ?15 -webkit-

6.1 -webkit-

21

min-content

46

222

No3 -moz- ?15 -webkit-

6.1 -webkit-

22

available No No3 -moz- ? ? ?
fill-available22 -webkit- No No ? ?6.1 -webkit-
fit-content

46

22 -webkit-

No3 -moz- No15 -webkit- 6.1 -webkit-
content-box No No No No No No
border-box No No No No No No
fill46 No ? ? ? ?
FeatureAndroid webviewChrome for AndroidEdge mobileFirefox for AndroidOpera AndroidiOS SafariSamsung Internet
Basic support Yes Yes Yes4 Yes Yes ?
Animatable ? ? ?16 ? ? ?
max-content4646 No ? ? ? ?
min-content4646 No ? ? ? ?
available ? ? No ? ? ? ?
fill-available4646 No ? ? ? ?
fit-content4646 No ? ? ? ?
content-box ? ? No ? ? ? ?
border-box ? ? No ? ? ? ?
fill4646 No ? ? ? ?

1. Supported as intrinsic.

2. Supported as min-intrinsic.

參考

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