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).
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.
