This is continuation for #5351.

The problem is that there's no way to render an asynchronously loaded component on SSR. My use-case for asynchronously loaded components is rendering JAM-stack site's content from JSON files. My JSON-files describe page content as blocks, that are different components (type) and their props (data).

I want to have top-level await support for component's <script> that will allow awaiting for promises before component's initialization is finished.

I've done an implementation for this, but changing initialization asynchronous is quite a breaking change. After the change it's not possible to initialize component with constructor, but instead we have to use a static async builder method Component.init(options). Not being able to initialize synchronously breaks custom elements. Component updating also becomes asynchronous so assignments to props don't get reflected to DOM synchronously. That will also break a lot of code.

If I make async initialization a compiler option, so that it doesn't trash backwards compatibility, would maintainers be willing to merge the changes? Is there any demand for this feature?