How does React 19 support custom elements differently from previous versions

Enhanced Prop Handling

1. Separation of Attributes and Properties: In React 19, props that correspond to properties on a custom element instance are assigned as properties during client-side rendering. If the prop is a primitive type (like `string`, `number`, or `true`), it is rendered as an attribute. Non-primitive types (like `object`, `symbol`, or `function`) are excluded from rendering. This approach allows for a more intuitive interaction with custom elements, aligning with how they are typically designed to function[3][4].

2. Server-Side Rendering (SSR) Improvements: During SSR, the same logic applies: primitive props are rendered as attributes, while non-primitive types are omitted. This ensures that the server-rendered output is consistent with what the client expects, reducing potential mismatches during hydration[4][5].

3. Scoped Custom Element Registries: React 19 supports scoped custom element registries, allowing developers to manage custom elements more effectively in micro-frontend architectures. This feature helps prevent version conflicts when multiple applications depend on different versions of the same custom element, enhancing modularity and compatibility[2].

4. Improved Performance Metrics: React 19 has achieved a 100% score on the Custom Elements Everywhere benchmark, a significant improvement over previous versions. This indicates better performance and compatibility with web components, addressing past criticisms regarding React's handling of custom elements[5].

These enhancements in React 19 not only streamline the integration of custom elements into React applications but also improve overall performance and developer experience, making it easier to work with web components in a React context.