width

/* <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;

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

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>