[Proposal] Stylesheet Observer

[bahrus]

It would be nice for a web component to be able to observe stylesheets as they are adopted or imported by the parent Shadow DOM scope or the root document, and pick and choose which ones to import.

So the basic shape would look like:

const observer = new StyleSheetObserver(options: StyleSheetObserverOptions);
observer.addEventListener('stylesheetadded', e => {
    console.log(e.stylesheet);
});
observer.addEventListener('stylesheetremoved', e => {
    console.log(e.stylesheet);
});
observer.observe(shadowRoot);
observer.observe(document);