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tools/stub-css.mjs

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+/**
+ * Registers the CSS stub for the test runner.
+ *
+ * Loaded with `node --import`, which has to happen before any test module is
+ * evaluated — a hook registered later cannot intercept an import that has
+ * already been resolved.
+ */
+import { register } from 'node:module';
+register('./css-hooks.mjs', import.meta.url);