Background colour is drawn over elements with negative z-index

[jdm]

<style>
html {
  background-color: #7D2663;
}

h1 {
  position: absolute;
  z-index: -1;
}
</style>
<h1>Web</h1>

Firefox:

Servo:

[pyfisch]

This happens only with backgrounds for the root element. If the background is part of any other element both browsers do not display the text.