max-width

max-width: 150px;

max-width: 20em;

max-width: 75%;

max-width: 20ch;

<section class="default-example" id="default-example">
  <div class="transition-all" id="example-element">
    Change the maximum width.
  </div>
</section>

#example-element {
  display: flex;
  flex-direction: column;
  background-color: #5b6dcd;
  height: 80%;
  justify-content: center;
  color: #ffffff;
}

max-width overrides width, but min-width overrides max-width.

<length>

Defines the max-width as an absolute value.

<percentage>

Defines the max-width as a percentage of the containing block's width.

none

No limit on the size of the box.

max-content

The intrinsic preferred max-width.

min-content

The intrinsic minimum max-width.

fit-content

Use the available space, but not more than max-content, i.e., min(max-content, max(min-content, stretch)).

fit-content(<length-percentage>)

Uses the fit-content formula with the available space replaced by the specified argument, i.e., min(max-content, max(min-content, argument)).

stretch

Limits the maximum width of the element's margin box to the width of its containing block. It attempts to make the margin box fill the available space in the containing block, so in a way behaving similar to 100% but applying the resulting size to the margin box rather than the box determined by box-sizing.

Note: To check aliases used by browsers for the stretch value and its implementation status, see the Browser compatibility section.

Ensure that elements set with a max-width are not truncated and/or do not obscure other content when the page is zoomed to increase text size.

• MDN Understanding WCAG, Guideline 1.4 explanations
• Understanding Success Criterion 1.4.4 | W3C Understanding WCAG 2.0

max-width = 
  none                                      |
  <length-percentage [0,∞]>                 |
  min-content                               |
  max-content                               |
  fit-content( <length-percentage [0,∞]> )  |
  <calc-size()>                             |
  <anchor-size()>                           

<length-percentage> = 
  <length>      |
  <percentage>  

<calc-size()> = 
  calc-size( <calc-size-basis> , <calc-sum> )  

<anchor-size()> = 
  anchor-size( [ <anchor-name> || <anchor-size> ]? , <length-percentage>? )  

<calc-size-basis> = 
  <size-keyword>  |
  <calc-size()>   |
  any             |
  <calc-sum>      

<calc-sum> = 
  <calc-product> [ [ '+' | '-' ] <calc-product> ]*  

<anchor-name> = 
  <dashed-ident>  

<anchor-size> = 
  width        |
  height       |
  block        |
  inline       |
  self-block   |
  self-inline  

<calc-product> = 
  <calc-value> [ [ '*' | / ] <calc-value> ]*  

<calc-value> = 
  <number>        |
  <dimension>     |
  <percentage>    |
  <calc-keyword>  |
  ( <calc-sum> )  

<calc-keyword> = 
  e          |
  pi         |
  infinity   |
  -infinity  |
  NaN        

In this example, the "child" will be either 150 pixels wide or the width of the "parent," whichever is smaller.

HTML

<div id="parent">
  <div id="child">
    Fusce pulvinar vestibulum eros, sed luctus ex lobortis quis.
  </div>
</div>

CSS

#parent {
  background: lightblue;
  width: 300px;
}

#child {
  background: gold;
  width: 100%;
  max-width: 150px;
}

Result

• min-width
• width
• max-inline-size
• max-block-size
• box-sizing
• Introduction to the CSS basic box model
• CSS box model module