There is a 403 (Forbidden) error and the Mapbox tiles will not display when viewing from Google Chrome (zoom in six times with the mouse wheel to view tiles). I'm using a Mapbox API access token and the app is hosted from Github. The app works fine from Firefox.
This won't be the solution, it will just point to the most probable cause of the 403 error.
Here are request headers from three different browsers, where Firefox and venerable IE11 worked OK and Chrome failed.
IE11 (works OK):
Accept: image/png, image/svg+xml, image/*;q=0.8, */*;q=0.5 Referer: https://underjollyroger.github.io/EarlyIllinoisLandcover/ Accept-Language: sl-SI User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; Trident/7.0; rv:11.0) like Gecko Accept-Encoding: gzip, deflate Host: api.mapbox.com DNT: 1 Connection: Keep-Alive Cache-Control: no-cache
Firefox (works OK):
Accept: image/webp,*/* Accept-Encoding: gzip, deflate, br Accept-Language: en-US,en;q=0.5 Connection: keep-alive Cookie: _ga=GA1.2.53209774.1591279471;…apbox.com-1591279471188-18008 Host: api.mapbox.com Referer: https://underjollyroger.github.io/EarlyIllinoisLandcover/ User-Agent: Mozilla/5.0 (Windows NT 6.1; W…) Gecko/20100101 Firefox/70.0
Chrome (fails with 403):
Accept: image/avif,image/webp,image/apng,image/*,*/*;q=0.8 Accept-Encoding: gzip, deflate, br Accept-Language: en-GB,en;q=0.9,en-US;q=0.8,sl;q=0.7 Cache-Control: no-cache Connection: keep-alive Host: api.mapbox.com Pragma: no-cache Referer: https://underjollyroger.github.io/ Sec-Fetch-Dest: image Sec-Fetch-Mode: no-cors Sec-Fetch-Site: cross-site User-Agent: Mozilla/5.0 (Windows NT 6.1; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/86.0.4240.198 Safari/537.36
What immediately stands out in Chrome headers are additional
Sec-Fetch-* settings which obviously impose some additional restrictions regarding CORS policy, and it seems like Mapbox server does not like that.
As suggested, I contacted Mapbox support and they solved the problem. It had to do with restricting the map tiles access token to specific URLS in my Mapbox account. https://docs.mapbox.com/help/how-mapbox-works/access-tokens/#adding-url-restrictions-to-access-tokens
It was pointed out that Chrome and Firefox had different referrer headers.
https://underjollyroger.github.io/EarlyIllinoisLandcover in the list of URLs already, but Chrome needed a more specific URL. So, after adding
https://underjollyroger.github.io to the list the tiles finally showed up in Chrome.