My name is Yves, I live in Switzerland (Vaud) near the Jura mountains.
OpenPisteMap test rendering with mapnik: Nordic pistes
From Jesh SVG proposal on 
Bottom left corner: difficulty. The 'freeride' tag and color exists, so I added it. From center to top right: backcountry, scooter, classic, skating, cl + sk, oneway, lit.
Some are not rendered yet, if difficulty or grooming is not tagged, plus I need ideas for pistes prepaired for 'winter hiking', sleigh and snowshoeing.
Now there is a grey line is difficutly is unknown or extreme (??) and quasi invisible difficulty-colored dots when grooming is unknown. I don't know if this sort of rendered FIXME is of use, but I think this could encourage mappers to improve their mapping.
On top right: added winter-hiking and piste:type=sleigh.
I try to have various rendering style with an automatic color for difficulties. Downhills went slightly transparent with white underline to differentiate them from others. Almost there, any idea for ice-skating ?
Template file here: 
I've prepare a few icons usefull for OPM, svg format.