MDN’s new design is in Beta! A sneak peek:



CSS 属性 flex-basis 指定了 flex 元素在主轴方向上的初始大小。如果不使用 box-sizing 来改变盒模型的话,那么这个属性就决定了 flex 元素的内容盒(content-box)的宽或者高(取决于主轴的方向)的尺寸大小。

适用元素flex items, including in-flow pseudo-elements
Percentagesrefer to the flex container's inner main size
计算值as specified, but with relative lengths converted into absolute lengths
Animation typea length, percentage or calc();
正规顺序the length or percentage before the keyword, if both are present


/* Specify <'width'> */
flex-basis: 10em;
flex-basis: 3px;
flex-basis: auto;

/* Intrinsic sizing keywords */
flex-basis: fill;
flex-basis: max-content;
flex-basis: min-content;
flex-basis: fit-content;

/* Automatically size based on the flex item’s content */
flex-basis: content;

/* Global values */
flex-basis: inherit;
flex-basis: initial;
flex-basis: unset;


width 值可以是一个数字后面跟着绝对单位例如 px, mm, pt; 该值也可以是一个百分数,那么这个百分数就是相对于其父弹性盒容器的宽或者高(取决于主轴方向)。负值是不被允许的。
基于 flex 的元素自动调整大小。
Note:Note that this value was not present in the initial release of Flexible Box Layout, and thus some older implementations will not support it. The equivalent effect can be had by using auto together with a main size (width or height) of auto.

Note: Brief history

  • Originally, "flex-basis:auto" meant "look at my width or height property".
  • Then, flex-basis:auto was changed to mean automatic-sizing, and "main-size" was introduced as the "look at my width or height property" keyword. It was implemented in bug 1032922.
  • Then, that change was reverted in bug 1093316, so "auto" once again means "look at my width or height property"; and a new 'content' keyword is being introduced to trigger automatic sizing. (bug 1105111 covers adding that keyword).


content | <'width'>



<ul class="container">
  <li class="flex flex1">1: flex-basis test</li>
  <li class="flex flex2">2: flex-basis test</li>
  <li class="flex flex3">3: flex-basis test</li>
  <li class="flex flex4">4: flex-basis test</li>
  <li class="flex flex5">5: flex-basis test</li>

<ul class="container">
  <li class="flex flex6">6: flex-basis test</li>


.container {
  font-family: arial, sans-serif;
  margin: 0;
  padding: 0;
  list-style-type: none;
  display: flex;
  flex-wrap: wrap;

.flex {
  background: #6AB6D8;
  padding: 10px;
  margin-bottom: 50px;
  border: 3px solid #2E86BB;
  color: white;
  font-size: 20px;
  text-align: center;
  position: relative;

.flex:after {
  position: absolute;
  z-index: 1;
  left: 0;
  top: 100%;
  margin-top: 10px;
  width: 100%;
  color: #333;
  font-size: 18px;

.flex1 {
  flex-basis: auto;

.flex1:after {
  content: 'auto';

.flex2 {
  flex-basis: -webkit-max-content;
  flex-basis: -moz-max-content;
  flex-basis: max-content;

.flex2:after {
  content: 'max-content';

.flex3 {
  flex-basis: -webkit-min-content;
  flex-basis: -moz-min-content;
  flex-basis: min-content;

.flex3:after {
  content: 'min-content';

.flex4 {
  flex-basis: -webkit-fit-content;
  flex-basis: -moz-fit-content;
  flex-basis: fit-content;

.flex4:after {
  content: 'fit-content';

.flex5 {
   flex-basis: content;

.flex5:after {
  content: 'content';

.flex6 {
  flex-basis: -webkit-fill-available;
  flex-basis: -moz-available;
  flex-basis: fill;

.flex6:after {
  content: 'fill/-webkit-fill-available/-moz-available';



Specification Status Comment
CSS Flexible Box Layout Module
Candidate Recommendation Initial definition


Feature Firefox (Gecko) Chrome Edge Internet Explorer Edge Opera Safari
Basic support 18.0 (18.0)[1]
22.0 (22.0)[2]
21.0-webkit (Yes) 11[3] 12 12.10 7.0-webkit[4]
auto 18.0 (18.0) 21.0 ? 11 12 12.10 7.0-webkit
content 未实现[5] 未实现 ? 未实现 12 未实现 未实现
Feature Firefox Mobile (Gecko) Android Edge IE Phone Opera Mobile Safari Mobile
Basic support ? ? (Yes) 未实现 12.10 未实现

[1] To activate flexbox support, for Firefox 18 and 19, the user has to change the about:config preference "layout.css.flexbox.enabled" to true. Firefox supports multi-line flexbox since Firefox 28.

[2] In addition to the unprefixed support, Gecko 48.0 (Firefox 48.0 / Thunderbird 48.0 / SeaMonkey 2.45) added support for a -webkit prefixed version of the property for web compatibility reasons behind the preference layout.css.prefixes.webkit, defaulting to false. Since Gecko 49.0 (Firefox 49.0 / Thunderbird 49.0 / SeaMonkey 2.46) the preference defaults to true.

[3] When a non-auto flex-basis is specified, Internet Explorer 10-11 (but not 12+) always uses a content-box box model to calculate the size of a flex item, even if box-sizing: border-box is applied to the element. See Flexbug #7 for more info.

[4] See Safari 7.0.

[5] See bug 1105111.



 此页面的贡献者: dongyu.zhang, fscholz, BenFei, Go7hic, Sebastianz, FredWe
 最后编辑者: dongyu.zhang,