The inline parts of {ib} splits are rendered as if they're elements unto themselves.

[Permutatrix]

This is noticeable in the rendering of margins, borders, and padding. Take this snippet:

<style>
  span {
    padding: 10px;
    border: 1px solid black;
    margin: 10px;
    background-color: #bbb;
  }
  div {
    padding: 1em 0;
  }
</style>
<span>
  A
  <div>b</div>
  C
  <div>d</div>
  E
</span>

Firefox renders the above like this:

Servo renders it like this:

My patch for #13982 exacerbates this problem, and it's polluting the test results. I'm going to take a crack at it. I'm hoping it'll be a relatively quick fix, but if it isn't, I suppose I'll just live with having to sift through broken reftests to find the relevant ones rather than getting sidetracked even further.