Right. For example S3 supports custom headers, as long as that header happens to start with "x-amz-meta-..." - and now your progress reporting is tied to your C

Right. For example S3 supports custom headers, as long as that header happens to start with "x-amz-meta-..." - and now your progress reporting is tied to your CDN choice! Not sure about you, but to me "XmlHttpRequest" in my request handling code feels less dirty than "x-amz-meta-". But to each their own I guess.