CSS width transition not working when originally setting width to fit-content

I expect it has something to do with the width: fit-content property not being a number, but I don't know how to fix it without using a fixed width. As you can see below, the divs don't expand smoothly when hovered over. Instead, they expand to 100% instantly.

* {
  font-family: sans-serif;
}

div:not(#container) {
  width: fit-content;
  background-color: green;
  color: white;
  margin-top: 1em;
  padding: 1em;
  transition-property: width;
  transition-duration: 1s;
}

#ease {
  transition-timing-function: ease;
}

#linear {
  transition-timing-function: linear;
}

#ease-in {
  transition-timing-function: ease-in;
}

#ease-out {
  transition-timing-function: ease-out;
}

#in-out {
  transition-timing-function: ease-in-out;
}

#container:hover div {
  width: 90%;
}

<html>
  <head>
    <title>CSS animations</title>
  </head>
  <body>
    <div id="container">
      <div id="linear">
        <p>
          Linear
        </p>
      </div>
      <div id="ease">
        <p>
          Ease
        </p>
      </div>
      <div id="ease-in">
        <p>
          In
        </p>
      </div>
      <div id="ease-out">
        <p>
          Out
        </p>
      </div>
      <div id="in-out">
        <p>
          In-out
        </p>
      </div>
    </div>
  </body>
</html>

Solution

Your assumption/expectation about width: fit-content seems reasonable to me. Until someone can explain better, you could use width: fit-content along with min-width: 0, then animate min-width. It's not perfect, since the animation begins at 0 (you may notice a slight delay)

* {
  font-family: sans-serif;
}

div:not(#container) {
  width: fit-content;
  min-width: 0;
  background-color: green;
  color: white;
  margin-top: 1em;
  padding: 1em;
  transition-property: min-width;
  transition-duration: 1s;
}

#ease {
  transition-timing-function: ease;
}

#linear {
  transition-timing-function: linear;
}

#ease-in {
  transition-timing-function: ease-in;
}

#ease-out {
  transition-timing-function: ease-out;
}

#in-out {
  transition-timing-function: ease-in-out;
}

#container:hover div {
  min-width: 90%;
}

<html>
  <head>
    <title>CSS animations</title>
  </head>
  <body>
    <div id="container">
      <div id="linear">
        <p>
          Linear
        </p>
      </div>
      <div id="ease">
        <p>
          Ease
        </p>
      </div>
      <div id="ease-in">
        <p>
          In
        </p>
      </div>
      <div id="ease-out">
        <p>
          Out
        </p>
      </div>
      <div id="in-out">
        <p>
          In-out
        </p>
      </div>
    </div>
  </body>
</html>