Minimal reproducer:

<style>
  .a {
    position: absolute;
  }
</style>
<pre>
<span class="a">1</span>  fo<span>o</span>
<span class="a">2</span>  fo<span>o</span>
<span class="a">3</span>  fo<span>o</span>
</pre>

Screenshots (Firefox left, Ladybird right):

This is a reduction from web content, particularly the following Wikipedia page:
https://en.wikipedia.org/wiki/Methods_of_computing_square_roots#Binary_numeral_system_(base_2)
Screenshots (Firefox left, Ladybird right):

I'd be happy to work on this myself, but I'm stuck: I guess that it's {Block,Inline}FormattingContext somehow, but I'm not sure whether these even are where the position is set.

Maybe InlineFormattingContext::run is supposed to set top of inline elements while running somehow? top is different between Firefox and Ladybird:

<style>
  .a {
    position: absolute;
  }
</style>
<pre>
<span class="a">1</span>  fo<span>o</span>
<span class="a">2</span>  fo<span>o</span>
<span class="a" id="three">3</span>  fo<span>o</span>
</pre>
<p id="result"></p>
<script>
    var computed = window.getComputedStyle(document.getElementById("three"))["top"];
    document.getElementById("result").textContent = computed;
</script>

Suggested tags: css, has repro, reduction of web content