max-content

/* 長さとして使用 */
width: max-content;
inline-size: max-content;
height: max-content;
block-size: max-content;
/* グリッドトラックで使用 */
grid-template-columns: 200px 1fr max-content;

<div id="container">
  <div class="item">Item</div>
  <div class="item">
    Item with more text in it which will overflow the fixed width box.
  </div>
</div>

#container {
  background-color: #8cffa0;
  padding: 10px;
  width: 200px;
}
.item {
  width: max-content;
  background-color: #8ca0ff;
  padding: 5px;
  margin-bottom: 1em;
}

<div id="container">
  <div>Item</div>
  <div>Item with more text in it.</div>
  <div>Flexible item</div>
</div>

#container {
  display: grid;
  grid-template-columns: max-content max-content 1fr;
  grid-gap: 5px;
  box-sizing: border-box;
  height: 200px;
  width: 100%;
  background-color: #8cffa0;
  padding: 10px;
}
#container > div {
  background-color: #8ca0ff;
  padding: 5px;
}