For ::first-letter https://drafts.csswg.org/css-pseudo-4/#application-in-css says

In CSS the first letter of a table-cell or inline-block cannot be the first letter of an ancestor element. [...] If an element is a list item (display: list-item), the ::first-letter applies to the first letter in the principal box after the marker.

But for ::first-line https://drafts.csswg.org/css-pseudo-4/#first-text-line only has the first part

The first line of a table-cell or inline-block cannot be the first formatted line of an ancestor element.

Should markers be excluded too? testcase

.inside {
  list-style-position: inside;
}
li::first-line {
  color: magenta;
}
li::first-letter {
  color: blue;
}

<ol>
  <li class="inside">inside</li>
  <li>outside</li>
</ol>

Chromium always excludes markers, Firefox includes them when they have inside positioning.

CC @MatsPalmgren