Reject document cloning and importing explicitly

[olavoasantos]

Problem

Document.cloneNode() and Document.importNode(document) entered the generic cloning path, which cannot deliberately construct a document node. Instead of declaring whether this unsupported operation was available, the polyfill leaked incidental internal TypeErrors.

Impact

Minor. Code that attempts to clone or import a document receives an implementation accident rather than a stable, recognizable DOM error, so it cannot reliably distinguish this unsupported operation from an unrelated programming failure.

Reproduction

const destination = new Window().document;

window.document.cloneNode(true);
destination.importNode(window.document, true);

Before this change, both calls failed through the generic clone implementation with incidental internal errors. They now throw a NotSupportedError with a clear operation-specific message; cloning and importing ordinary document descendants remains supported.

Change

Detect document nodes before entering the generic clone path. Both Document.importNode() and cloneNode() now reject them with a named NotSupportedError, using DOMException where available and an Error with the same name as a fallback.

Tests

Adds packages/polyfill/source/tests/document-clone-import-contract.test.ts, covering shallow and deep document cloning/importing, the error name and messages, the no-DOMException fallback, and successful cloning/importing of document.documentElement.

Stack

• Stack: Cloning and importing (#700), layer 2 of 2
• Base: preserve-clone-namespaces
• Review after PR Preserve namespaces and template content when cloning nodes #683.

Validation

Fresh GitHub CI on restacked head 8d31750 passes:

• lint;
• type-check and build;
• unit tests with coverage;
• bundle-size checks;
• Playwright end-to-end tests;
• the classified Web Platform Test suite.