What browsers suppor it now?
What browsers are thinking about it? Why are they hesitating?
Browsers must support the first three Interledger layers. Is SPSP or BTP the missing layer?
An Interledger-supporting implementation (say, a browser) must support the first three Interledger layers. That means that most of Interledger must be created, and not just the API. This is a lot of work, and that is one of the reasons browsers are not taking up Web Monetization quickly.
A Gecko implementation has been written, but it does not include the Interledger stack, and is dependent on the Coil extension. A user must have the Coil extension installed in order to access the 'native' implementation.
Possible way out: let browsers implement just the API and send encrypted payment requests to a neutral third-party server over regular HTTP, with that server being tasked with getting the payment onto Interledger, and sending a receipt or error message back to the browser. This allows browser vendors to skip implementing the Interledger layer.
payment pointer points to JSON file with address, which may change. Browsers must implement STREAM and IP-like layer SPSP does roughly the same as IP. Sends packages to address specified in JSON. BTP not necessary: send to Coil and then send it on. If they did they'd become ledgers. Only apps that are ledgers shoudld support BTP. connection point with the ledger. Ledger does not need to be Interledger. Browsers need to support STREAM and SPSP. ILPv4. Money is binary. SPSP is envelope.