Bug: Properties are not passed to Custom Elements that extend built-in elements

[edoardocavazza]

React version: 19.0.0

Steps To Reproduce

1. Define a Custom Element x-custom-link that extends the a element
2. Render the custom element using the is attribute: <a is="x-custom-link"></a>
3. Pass a property to the element

Link to code example: https://codesandbox.io/p/sandbox/musing-shadow-ttt8hc

The current behavior

Properties are correctly passed to autonomous custom elements but not to those that extend builtin elements.

The expected behavior

Properties are passed to custom elements that extend builtin elements as well.

[edoardocavazza]

I also assume one of those tests is wrong, since <div is="custom-element"> should behave like <custom-element>:

• https://github.com/facebook/react/blob/main/packages/react-dom/src/__tests__/ReactDOMServerIntegrationAttributes-test.js#L713
• https://github.com/facebook/react/blob/main/packages/react-dom/src/__tests__/ReactDOMServerIntegrationAttributes-test.js#L719