We make JSX configuration a per-val setting by requiring a jsxPragma: https://docs.deno.com/runtime/manual/advanced/jsx_dom/jsx
But most code doesn’t do that and relies on a project-wide jsx setting. So, to provide better DX for missing JSX pragmas, we use this CodeMirror extension that detects the lack of a pragma and the presence of JSX syntax, by using CodeMirror’s existing syntax tree.