The coordinates in the indicator area has an extra right margin in legacy output which is missing for Parsoid. Inspecting the HTML reveals that in the case of Parsoid, the coordinates looks like this: <span id="mw..", data-x-id="coordinates">...</span>. So, Parsoid thinks that the coordinates id is duplicated and dedupes it. So, this means that the CSS rules #coordinates doesn't apply to Parsoid output which is the cause of the diff.
However, if you look at the full page source, you see this: <meta typeof="mw:Extension/indicator mw:Transclusion" about="#mwt3" id="mwCw" data-mw='{"name":"indicator","attrs":{"name":"coordinates"},"body":{..},"html":"<span id=\"coordinates\">...</span>","parts":[{"template":{"target":{"wt":"Coord","href":"./Modèle:Coord"},"params":{"1":{"wt":"40"},"2":{"wt":"-105"},"3":{"wt":"scale:20000000"},"display":{"wt":"title"}},"i":0}}]}'/><link rel="mw:PageProp/Category" href="./Catégorie:Article_géolocalisé_aux_États-Unis" about="#mwt3"/> (a bunch of content elided from that).
So, you see that the meta tag's data-mw has embedded HTML that is a copy of the indicator HTML that is now part of the main body. And, running deduping across the indicator and the main doc causes it to conflict with itself.
This is a false positive id conflict and should be handled. This is likely an issue in the OTP in core.