The label for the preference has a newline in it (because it wants to be a heading and a description, and that's not supported in HTMLForm).
Because of the new lines, the label element includes <p> tags, which isn't only semantically wrong, but also leads to misalignment of help tooltip indicator.
Given that this is the only item in the “Reading preferences” section (even though everything in this tab is a preference that affects reading in some way), maybe we could re-label the section to Page Previews and use the description as the label? This would retain the current two-message ability, but might interfere with future plans for this feature?
(There's also T174726: Page Preview preference should be a checkbox about this preference being a radio button and not a checkbox, which is confusingly inconsistent for users and bloats the code a fair bit, but that doesn't need fixing at the same time.)