In #4448 (comment) we resolved that ::marker should have white-space: pre in UA origin. That is needed for outside markers, and we decided to keep it simple and also apply it to inside markers.

But consider this case:

<ol style="list-style-position: inside">
  <li>Text</li>
  <li>
    Text
  </li>
</ol>

For block container elements like <li>, leading and trailing space doesn't matter. So just looking at the HTML, some authors may think that both Text will be aligned equally.

However, the ::marker is inserted at the very beginning of the <li>, and since it's inside, it behaves as a normal inline. So the leading space is no longer at the beginning of the line, and is not trimmed in §4.1.2.1.

Since the ::marker has a trailing space, both list items could still look the same via §4.1.1:

Any collapsible space immediately following another collapsible space [...] is collapsed

But that doesn't apply if the ::marker has white-space: pre. So the 1st list item ends up with 1 space and the 2nd list item with 2 spaces.

What behavior do we want here? We don't have interoperability:

• In Chromium (both legacy and LayoutNG), WebKit and old Edge, the texts are aligned:

  

• In Firefox, they aren't:

  

So I guess we can choose among:

1. Keep white-space: pre. Firefox is correct, changing Chromium will be trivial.
2. Keep white-space: pre but add some magic that if a text ::marker has a trailing space and is followed by collapsible spaces, then these collapsible spaces are removed, even if the ::marker space is not collapsible. This will keep Chromium's behavior but implementing this magic would be annoying.
3. Say that inside markers don't get assigned white-space: pre in UA origin. This is close to what Chromium does right now. But I guess doing this properly would need pseudo-classes like ::marker:inside and ::marker:outside, even if just for internal use, but that would create a circularity if in the future we allow setting list-style-position in the ::marker itself.

I lean towards 1.