The attached testcase causes a panic in m-c rev c22502562670 with stylo enabled by pref. thread 'StyleThread#1' panicked at 'Unstyled element shouldn't be traversed during animation-only traversal', /home/worker/workspace/build/src/servo/components/style/data.rs:342 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: style::data::ElementData::restyle_kind_for_animation 7: style::data::ElementData::restyle_kind 8: style::traversal::compute_style 9: style::traversal::recalc_style_at 10: <style::gecko::traversal::RecalcStyleOnly<'recalc> as style::traversal::DomTraversal<style::gecko::wrapper::GeckoElement<'le>>>::process_preorder 11: style::parallel::traverse_nodes 12: style::parallel::traverse_dom::{{closure}}::{{closure}} 13: rayon_core::scope::Scope::execute_job_closure::{{closure}} 14: <std::panic::AssertUnwindSafe<F> as core::ops::FnOnce<()>>::call_once 15: std::panicking::try::do_call 16: <unknown> Redirecting call to abort() to mozalloc_abort

This one is bug 1383001, but a much nicer test-case as usual :-). Will push it, thanks!