In CodeMirror, a warning is shown if you use more modern CSS. For example: li:has(#checkprevcheckout) { display: none; } This is valid, but CodeMirror displays a warning. Same for nested CSS: .thing1 { .thing2 { display: none; } } Also valid, but it recreates warnings that would make you think otherwise.
Do we need to update to a newer version of codemirror? V6 appears to support this modern CSS