Interactive (e.g. navigation, attribution) element have z-index 10.
Overlayed elements (slider, route bubble) have z-index 20.
Filling elements have z-index -1 to 9.
Cover have z-index 100.
Special elements (debug) have z-index 50.
Do not change z-index in server cssnano plugin (z-index are not
changed when built from npm script).