Why is hyphenation not working with lang en

Question

What is wrong with this code that hyphenations don't work for lang="en"?

h1 {
  font-size: 2em;
  hyphens: auto;
}

div {
  max-width:200px;
  background: aqua;
}

<div>
  <h1 lang="en">Kindofverylongheadlineevenbiggerthanimagined</h1>
  <h1 lang="en">Internationalization</h1>
  <h1 lang="de">Kindofverylongheadlineevenbiggerthanimagined</h1>
</div>