Wireframe view is nice, but in this particular case, the dotted lines of standard JOSM let me see in the gaps the line I have to map and attain an extraordinary precision (hoping that the trace itself is precise).
I have solved the square sizes this way (copied without tuning from wiki/Styles, thanks Felis) :
node|z1-16
{ symbol-shape: square; symbol-fill-opacity: 0; symbol-size: 1; z-index:-1}
node|z17-
{ symbol-shape: square; symbol-stroke-color: navy; symbol-fill-color: rosybrown; symbol-size:8; z-index:1}
The other problem that emerged is that the WMS server is designed for black (and colors) on white but that JOSM is designed for white (and colors) on black. Hence the black lines from the server are invisible on black and some colors are hardly visible. I have partially solved this problem as follows:
canvas { background-color: #404040; }
together with setting the server's background transparent.
My WMS colors are OK but one. Whichever shade of gray I choose, there will always be some standard JOSM colors that will display less than optimally. (And, BTW, I wonder how the lines JOSM chooses to be black are supposed to display).
The 'invert colors' CSS here above is in fact tweaking the JOSM color channels.
If a similar CSS could be applied to a particular server source, that would be the best way to improve the server display on black or gray while trying to keep its natural colors.
In fact, it's not a matter of color but of intensity: only the least intense tones must change.
Another possibly easy modification would be to extend the opacity slider to negative numbers, for which the server colors would be inverted. This would certainly solve the visibility issue amusingly, but would not keep the server's natural colors (yellow rivers and red forests). Unless the slider did not invert the colors but tweak the least intense tones again.
This might be brought to the attention of the CSS development.
Thanks for your attention.
