The error is already fixed in recent JOSM 13190. The validator only shows it as information, I agree that highway should be treated different to other tags.

Hi!
I've fixed several ways with typo highway=unclassifed to
highway=unclassified here:
changeset/54531564
Please watch out to use the correct spelling ;-)

Hi!
I've removed highway=Residential from the buildings in this changeset. Maybe you meant building=residential?

Hi!
I've changed some highway=unclasified
to unclassified (note the double s).
Please make sure to use the correct spelling, JOSM 13170 sometimes forgets all highway types.

Hi!
I've removed highway=* and bridge=yes from way/242893994
What special information did you try to map with them?

Hi!
I've removed highway=water from this way:
way/539626954
What did you try to map with this tag?

Hi and welcome to OSM!
I've noticed this changeset because of the unusual tag highway="tertiary highway"
used on a building. What does it mean?
There is also ele=5.6, I assume you meant the height of the building (height=5.6)
Instead of name=house please use building=house. The name tag should only be used for objects which have a name, it should not describe the object.
There is also a highway=tertiary named "Tertiary Road" next to the building.
The road looks more like a path or track like the other roads nearby.
Hope this helps, happy mapping!

Moin!
Du hast hier highway=street_lamp an einen Parkplatz gehängt. Meintest Du lit=yes ?
way/543274265
Mit highway=street_lamp werden die einzelnen Lampen als Knoten erfaßt.

Hi!
You add highway=street_lamp to a closed way with amenity=parking. I assume you meant lit=yes ?
The tag highway=street_lamp is meant for the nodes where a single lamp is located.

Moin!
Das Tag highway=mini_roundabout ist nur für knoten gedacht, wenn ein (geschlossener) Weg gemalt wird, dann passt entweder junction=roundabout oder evtl. junction=circular und das highway tag sollte hier wohl highway=residential sein.
way/486122334

Hi!
Please check:
What does highway=mini_roundabout mean on these two ways?
way/545077392
way/545077384
I think the ways look like highway=unclassified, but where is the roundabout?

Moin!
Ich vermute, hier sollte es heigth=5 anstelle von highway=5 sein ?
way/532286546

Excuse mon francais.
Je suggère d'utiliser highway=cycleway.
highway=trak est une faute d'orthographe
(trak -> track).

OK, two points
1) trak is probably a typo for track
2) a track is meamt for agricultural vehicles, but this way has an explicit agricultural=no.
I think what you want to describe is a highway=cycleway, right?

Hi!
Please check highway=turning_circle on this way way/173271995.
I guess you wanted to change a different object?

Hi!
I've noticed this changeset because of the typo highway=residental (missing 2nd "i").
I assume that JOSM has saved this wrong spelling and uses it whenever you start typing r e s , so please watch out to correct it. I've changed 23 ways here:
changeset/54325730

Yes, the wiki is not very clear about the destinction of unclassifed and residential, and I am also often unsure which is better when mapping in Japan.
When you follow this wiki: highway=* for unclassified it says
"The least most important through roads in a country's system – i.e. minor roads of a lower classification than tertiary, but which serve a purpose other than access to properties. Often link villages and hamlets."
The given example serves to access properties, doesn't it?

Hi!
I noticed this changeset because of the typo highway=u here:way/543539978
I changed it to residential.
I wonder why you map the residential roads with highway=unclassified?

Hi!
Why did you change the tag highway=path to highway=trak
on this way :way/69082540 ? This tag is only used here and the previous value seems to be appropriate.

Ich habe das unübliche highway=stairs nach highway=steps geändert.