[endgame]

That's a good trick for JSON-based AWS APIs, but the S3 API is all XML AFAIK. I had to fix some signing bugs in the unofficial Haskell AWS SDK, and found it rather fiddly and a fair bit more than 30 LoC. Compared to standard AWS SigV4, S3 is also a special case in a few ways (request chunking, support for presigned PUTs with unsigned payloads, etc).

[capableweb]

Ah, good point, but of course the browser also ships with APIs for dealing with XML, not doing so would be weird as HTML and the XML structure a pretty fundamental part of the web. See XMLHttpRequest, XMLSerializer and DOMParser.