Currently, defaults are always considered, with URL parameters overriding them.
However, we would like to make URLs portable spatially, temporally, and interpersonally, so if urlversion=2 is specified, it should ignore defaults for hide_unselected_if_any and string_options. Instead, the de-minimis default will be "don't hide" (false or ALL, depending on type). (Others like namespace and tag already have a default of "don't hide").