What requirements must a micropayment solution fulfil to be considered as the de-facto standard for the web? Where do current solutions fall short? Which are the most promising solutions today?
I certainly remember using Flattr a long time ago — what ever became of that? It seems that "Donate Bitcoin", Patreon, PayPal and "Buy me a coffee" (https://www.buymeacoffee.com) are the most popular methods of supporting projects on the web these days. However these have always felt like a stop-gap solution compared to being able to, for example, click a button and have your browser send a payment of $0.01 to unlock an article.
Brave Browser (https://brave.com) seems to be a project which crops up frequently, although it prescribes a very specific model (use a specific browser, voluntarily view privacy-respecting ads, earn tokens and distribute tokens to enrolled websites that you visit). Is it likely that the Basic Attention Token (the "currency" that enables the interaction described above) will be included with other browsers?
Then of course there is the Bitcoin Lightning Network, which sidesteps some of the limitations of on-chain transactions. Strike (https://beta.strike.me) seems to be the missing piece, which provides an end-to-end solution for sending fiat currency by using the Bitcoin Lightning Network as the underlying payment transport.
There is also Stellar, which it seems is more concerned with providing a method of sending payments, than being its own currency, although the native currency "Lumens" are still required to complete a transaction (as I understand).
Stepping outside of the cryptocurrency world, there is also GNU Taler (https://taler.net/en/), a project to create a privacy-preserving payment system using existing banking infrastructure.