"medium" is valid. "Medium" is invalid.

Changing OSM data to contain redundant info because some specific map site/style doesn't display desired info is https://wiki.openstreetmap.org/wiki/Tagging_for_the_renderer and is considered bad practice.

In OSM first and foremost the data must be accurate. How it is displayed is up to each map style. If that map style is missing some critical information, that style should be updated, not OSM data mangled to fit that incomplete style.