This causes a crash: Assertion failure: 0 <= aStyle && aStyle < 18 (Require a valid builtin style constant), at stylo/layout/style/CounterStyleManager.cpp:2063 #01: Gecko_SetListStyleType (stylo\layout\style\servobindings.cpp:637) #02: style::properties::substitute_variables_list_style_type<closure> (stylo\servo\components\style\lib.rs:154) #03: style::properties::longhands::list_style_type::cascade_property (stylo\servo\components\style\lib.rs:154) #04: style::properties::apply_declarations::{{closure}}<closure,core::iter::FlatMap<core::slice::Iter<(style::rule_tree::StyleSourceGuard, style::properties::declaration_block::Importance)>, core::iter::FilterMap<core::iter::Rev<core::slice::Iter<(style::prope (stylo\servo\components\style\lib.rs:154) #05: style::gecko_properties::ComputedValues::do_cascade_property<closure> (stylo\servo\components\style\lib.rs:160) #06: style::properties::apply_declarations<closure,core::iter::FlatMap<core::slice::Iter<(style::rule_tree::StyleSourceGuard, style::properties::declaration_block::Importance)>, core::iter::FilterMap<core::iter::Rev<core::slice::Iter<(style::properties::Proper (stylo\servo\components\style\lib.rs:154) #07: style::properties::cascade (stylo\servo\components\style\lib.rs:154) #08: style::matching::PrivateMatchMethods::cascade_node_pseudo_element<style::gecko::wrapper::GeckoElement> (stylo\servo\components\style\matching.rs:430) #09: style::matching::MatchMethods::cascade_node<style::gecko::wrapper::GeckoElement> (stylo\servo\components\style\matching.rs:769) #10: style::traversal::compute_style<style::gecko::wrapper::GeckoElement,style::gecko::traversal::RecalcStyleOnly> (stylo\servo\components\style\traversal.rs:488) #11: style::traversal::recalc_style_at<style::gecko::wrapper::GeckoElement,style::gecko::traversal::RecalcStyleOnly> (stylo\servo\components\style\traversal.rs:396) #12: style::gecko::traversal::{{impl}}::process_preorder (stylo\servo\components\style\gecko\traversal.rs:32) #13: style::parallel::traverse_nodes::{{closure}}<style::gecko::wrapper::GeckoNode,style::gecko::traversal::RecalcStyleOnly> (stylo\servo\components\style\parallel.rs:145) #14: rayon::scope::{{impl}}::execute_job_closure::{{closure}}<closure> (stylo\third_party\rust\rayon\src\scope\mod.rs:316) #15: std::panic::{{impl}}::call_once<(),closure> (C:\bot\slave\stable-dist-rustc-win-msvc-64\build\src\libstd\panic.rs:254) #16: std::panicking::try::do_call<std::panic::AssertUnwindSafe<closure>,()> (C:\bot\slave\stable-dist-rustc-win-msvc-64\build\src\libstd\panicking.rs:352) #17: _rust_maybe_catch_panic[stylo\obj-firefox-stylo\dist\bin\xul.dll +0x6fde1c] Hit this crash in test_value_cloning.html. It is because lower-alpha and upper-alpha are listed in list.mako.rs which is supported as custom counter style in gecko.