width

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;

<div class="grid">
  <div class="col">
    <div class="cell">
      &lt;length&gt; values
      <p class="w1">width: 150px</p>
      <p class="w2">width: 20em</p>
      <p class="w3 warning" title="this feature is experimental and might not work in your browser">width: 20em content-box</p>
      <p class="w4 warning" title="this feature is experimental and might not work in your browser">width: 20em border-box</p>
    </div>
    <div class="cell">
      &lt;percentage&gt; values
      <p class="w5">width: 75%</p>
      <p class="w6 warning" title="this feature is experimental and might not work in your browser">width: 75% content-box</p>
      <p class="w7 warning" title="this feature is experimental and might not work in your browser">width: 75% border-box</p>
    </div>
    <div class="cell">
      Keyword values
      <p>width: auto</p>
      <p class="w8 warning" title="this feature is experimental and might not work in your browser">width: max-content</p>
      <p class="w9 warning" title="this feature is experimental and might not work in your browser">width: min-content</p>
      <p class="w10 warning" title="this feature is experimental and might not work in your browser">width: available</p>
      <p class="w11 warning" title="this feature is experimental and might not work in your browser">width: fit-content</p>
    </div>
  </div>
</div>

html,body {
  height: 100%;
  box-sizing: border-box;
}

.grid {
  width: 100%;
  height: 100%;
  display: flex;
  background: #EEE;
  font: 1em monospace;
}

.col {
  display: flex;
  flex: 1 auto;
  flex-direction: column;
}

.cell {
  margin: .5em;
  padding: .5em;
  background-color: #FFF;
  overflow: hidden;
  text-align: left;
  font-style: italic;
}

p {
  font-size: 1rem;
  font-style: normal;
  background: #E4F0F5;
  padding: .5em;
  margin: .5em;
}

.warning {
  background: #E4E4E4;
}

.warning:before {
  content: "⚠️ ";
}

/*  values */
.w1 { width: 150px; }
.w2 { width: 20em; }
.w3 { width: 20em content-box; }
.w4 { width: 20em border-box; }

/*  value */
.w5 { width: 75%; }
.w6 { width: 75% content-box; }
.w7 { width: 75% border-box; }

/* Keyword values (mostly experimental) */
.w8 { width: max-content; }
.w9 { width: min-content; }
.w10 { width: available; }
.w11 { width: fit-content; }

The width property is specified as either:

• one of the following keyword values: available, min-content, max-content, fit-content, auto.
• a <length> or a <percentage>. This may optionally be followed by one of the following keywords: border-box, content-box.

<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

auto | <length> | <percentage> | min-content | max-content | fit-content(<length-percentage>)
where 
<length-percentage> = <length> | <percentage>

p.goldie {
  background: gold;
}

<p class="goldie">The Mozilla community produces a lot of great software.</p>

.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>

.percent {
  width: 20%;
  background-color: silver;
  border: 1px solid red;
}

<div class="percent">Width in percentage</div>

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>

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>

• box model, height, box-sizing, min-width, max-width