Checkboxes are missing checked attribute when serialized

[Tsubasa1218]

Describe the bug
I want to bring up #1023 again, as the original behaviour described was not the one implemented in the fix.

When a Input element with type Checkbox is serialized in React tests, the checked="" is not being added.

To Reproduce

I've created this repo that showcases the issue: https://github.com/Tsubasa1218/happy-dom-checkbox.

The app only renders an Input[type="checkbox"] with checked={true}. I then create a snapshot test for both Happy DOM and JSDOM. You can check the snapshots here: https://github.com/Tsubasa1218/happy-dom-checkbox/tree/main/src/__snapshots__

Basically, the JSDOM output has checked="" but the Happy DOM doesn't.

I have tested that the checkbox is actually checked.

Expected behavior
Serialized output for checkboxes includes checked="" when the checkbox is checked.

Screenshots
This is the Happy DOM result

And this is the JSDOM result

Device:

• OS: MacOS 15.1
• Browser: Vitest
• Version: 2.1.4
• Happy DOM version: 15.10.1
• JSDOM version: 25.0.1

Additional context

I've been debugging the serialization step in our app, and I can see that the checkbox does not have the checked property in the namedItems map. This in XMLSerializer.js

This is the real test; Not much different from the repo I provided.

I haven't been able to find the place where these properties are being added.

Thanks for the help. If there is anything I can help with, let me know :)

[Tsubasa1218]

Looking at it more, it looks like the problem is when React DOM creates the node, resulting in the Input Node having the attributes I shared above.

I don't really know where the elements are being created, so any help would be appreciated (or a fix...)

Thanks again!