Error:

No route found for the specified format html. Supported formats: json, xml. in Drupal\Core\Routing\RequestFormatRouteFilter->filter() (line 65 of /var/www/html/web/core/lib/Drupal/Core/Routing/RequestFormatRouteFilter.php

Solution:

You probably encountered this error while trying to setup a view with REST export. Under 'format' click on 'settings' next to 'serialzer' and check 'json'. 

serializer