Clarify how to handle using sub resource web bundles when browsers support it, and traditional bundlers when they dont

Title is a bit verbose but gets the point across.

With script tags, when module support was added, nomodule was also added so you could have both in your markup and the browsers that knew modules loaded modules and intentionally didn't load nomodules, I feel a similar pattern is needed for WebBundles.

In the chromium Origin Trial, it is mentioned that its possible in JS to do feature detection for this, but that would require having web app developers to effectively polyfill webbundles ourselves and only add the JS bundle script tag at runtime once feature detection has happened, increasing the size of the html body and adding latency when instead this feature can ship with polyfill mechanisms baked in.

Start by reviewing the Chromium Origin Trial guidance mentioned in the issue, especially the JavaScript feature-detection approach. Clarify whether WebBundles should provide a declarative fallback mechanism comparable to module/nomodule, and document the agreed browser and bundler behavior as the definition of done.