The attached testcase causes a panic in m-c rev 68046a58f829 thread 'StyleThread#1' panicked at 'called `Option::unwrap()` on a `None` value', /checkout/src/libcore/option.rs:329 stack backtrace: 0: std::sys::imp::backtrace::tracing::imp::unwind_backtrace 1: std::sys_common::backtrace::_print 2: std::panicking::default_hook::{{closure}} 3: std::panicking::default_hook 4: std::panicking::rust_panic_with_hook 5: std::panicking::begin_panic 6: std::panicking::begin_panic_fmt 7: rust_begin_unwind 8: core::panicking::panic_fmt 9: core::panicking::panic 10: <core::option::Option<T>>::unwrap 11: <style::values::specified::color::Color as style::values::computed::ToComputedValue>::to_computed_value 12: <style::properties::longhands::color::SpecifiedValue as style::values::computed::ToComputedValue>::to_computed_value 13: style::properties::longhands::color::cascade_property 14: style::properties::apply_declarations 15: style::properties::cascade 16: <style::style_resolver::StyleResolverForElement<'a, 'ctx, 'le, E>>::cascade_style 17: <style::style_resolver::StyleResolverForElement<'a, 'ctx, 'le, E>>::resolve_primary_style 18: <style::style_resolver::StyleResolverForElement<'a, 'ctx, 'le, E>>::resolve_style 19: <style::style_resolver::StyleResolverForElement<'a, 'ctx, 'le, E>>::resolve_style_with_default_parents::{{closure}} 20: style::style_resolver::with_default_parent_styles 21: <style::style_resolver::StyleResolverForElement<'a, 'ctx, 'le, E>>::resolve_style_with_default_parents 22: style::traversal::compute_style 23: style::traversal::recalc_style_at 24: <style::gecko::traversal::RecalcStyleOnly<'recalc> as style::traversal::DomTraversal<style::gecko::wrapper::GeckoElement<'le>>>::process_preorder 25: style::parallel::traverse_nodes 26: style::parallel::traverse_nodes 27: style::parallel::traverse_dom::{{closure}}::{{closure}} 28: rayon_core::scope::Scope::execute_job_closure::{{closure}} 29: <std::panic::AssertUnwindSafe<F> as core::ops::FnOnce<()>>::call_once 30: std::panicking::try::do_call 31: <unknown> Redirecting call to abort() to mozalloc_abort

Nice testcase! (You're making finding out bugs much easier, thanks!) This should be bug 1381233, which is about to land.