HTMLTemplateElement.insertBefore() redirects to template.content instead of adding direct children

[ianlet]

Describe the bug
Calling insertBefore() on an HTMLTemplateElement appears to redirect the node to template.content instead of adding it as a direct child. This differs from browser behavior where insertBefore() adds direct children to the template element itself.

To Reproduce

const template = document.createElement('template');
const div = document.createElement('div');
template.insertBefore(div, null);

console.log(template.childNodes.length);         // happy-dom: 0
console.log(template.content.childNodes.length); // happy-dom: 1

Expected behavior
Based on Firefox (and jsdom), insertBefore() should add the node as a direct child:

console.log(template.childNodes.length);         // Expected: 1
console.log(template.content.childNodes.length); // Expected: 0

Firefox console output

template.insertBefore(div, null);
console.log(template.childNodes.length); // Output: 1
console.log(template.content.childNodes.length); // Output: 0

Device:

• OS: Linux
• happy-dom version: 20.0.11
• Compared against: Firefox, jsdom 26.1.0

Additional context
This may be an intentional design choice rather than a bug - perhaps enforcing the template content model more strictly. However, it causes compatibility issues with libraries that use template elements as generic containers for temporary DOM storage (e.g., Qwik's VirtualElementImpl), which work fine in real browsers.

We discovered this while trying to add support for happy-dom in Qwik Testing Library. Here's the workaround we implemented in the meantime: Add happy-dom support for Qwik Testing Library