Based on https://github.com/nodebox/opentype.js/issues/292 it appears that Zilla Slab uses GPOS/GSUB lookups that are intended for very large fonts, which is a bit puzzling as Zilla is pretty much a standard Latin-only font and shouldn't need to use these special purpose lookups. In fact, doing so may lead to parse failures in "simpler" or older engines, so it might be worth changing the UFO compilation such that normal GPOS/GSUB lookups are used.
Based on https://github.com/nodebox/opentype.js/issues/292 it appears that Zilla Slab uses GPOS/GSUB lookups that are intended for very large fonts, which is a bit puzzling as Zilla is pretty much a standard Latin-only font and shouldn't need to use these special purpose lookups. In fact, doing so may lead to parse failures in "simpler" or older engines, so it might be worth changing the UFO compilation such that normal GPOS/GSUB lookups are used.