In this example, the flex-grow and flex-shrink properties are both set to 1 on all three items, indicating that the flex item can grow and shrink from the initial flex-basis.

The demo then changes the flex-basis on the first item. It will then grow and shrink from that flex-basis. This means that, for example, when the flex-basis of the first item is 200px, it will start out at 200px but then shrink to fit the space available with the other items being at least min-content sized.

The image below shows how the Firefox Flexbox Inspector helps you understand the size items become:

/* Specify <'width'> */
flex-basis: 10em;
flex-basis: 3px;
flex-basis: 50%;
flex-basis: auto;

/* Intrinsic sizing keywords */
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: revert;
flex-basis: revert-layer;
flex-basis: unset;

The flex-basis property is specified as either the keyword content or a <'width'>.

<'width'>

Any of the following units:

• <length> sets an absolute value
• <percentage> sets a percentage of the width or height of a containing block's content area
• auto uses the value of the width in horizontal writing mode, and the value of the height in vertical writing mode; when the corresponding value is also auto, the content value is used instead
• max-content sets the intrinsic preferred width
• min-content sets the intrinsic minimum width
• fit-content sets the maximum possible size of a containing block's content area, bounded by the min-content and max-content values, and calculated based on the content of the current element

content

Indicates automatic sizing, based on the flex item's content.

Note: 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.

• 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 Firefox bug 1032922.
• Then, that change was reverted in Firefox 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. (Firefox bug 1105111 covers adding that keyword).

flex-basis = 
  content    |
  <'width'>  

HTML

<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>

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

CSS

.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: 14px;
  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: 12px;
}

.flex1 {
  flex-basis: auto;
}

.flex1::after {
  content: "auto";
}

.flex2 {
  flex-basis: max-content;
}

.flex2::after {
  content: "max-content";
}

.flex3 {
  flex-basis: min-content;
}

.flex3::after {
  content: "min-content";
}

.flex4 {
  flex-basis: fit-content;
}

.flex4::after {
  content: "fit-content";
}

.flex5 {
  flex-basis: content;
}

.flex5::after {
  content: "content";
}

Results

• CSS Flexbox Guide: Basic Concepts of Flexbox
• CSS Flexbox Guide: Controlling Ratios of flex items along the main axis
• width