I've never been a fan of user agent sniffing because of the unpredictability of the user agents, I prefer feature detection, which was the initial intention of the two script tags.
I like that there's a community supported project in Node, but that doesn't sort things for other language ecosystems or staticly rendered sites either. I admit, that when the worst case is the fallback to ES5 transpiled code, it's not the worst thing and the potential benefits probably outweigh my concern.
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