Since it took me quite some time to figure out, and it 'breaks CSS classes' .
(which I also have seen in some add-ons I can not remember)
Classes constructed 'with python' can turn an extra space into a dot, so:
> output += '<li class="my-class another class
ends up like
.myclass. .another . class
in html (notice the . after my class)
Which means that things like
.myclass.anotherclass
will not work