This is pretty cool: https://github.com/bcoin-org/bcoin
(browser version, running: http://bcoin.io/browser.html)
I’ve been thinking how a simple ecommerce site (say, selling coffee?) would make BitCoin an available payment method on their website.
From looking at it, the best approach would be to call the API of the official BitCoin client to validate transactions etc… This fits the bill nicely and opens up the possibility of not needing any kind of “backend” API to call, but rather just have it as a bunch of front-end JS code talking to each other in the same browser tab.