HTML is about the formatting of a published work. That is a PRODUCT, a public good that is free to all members of the public who can access it.
Payment is something that is arranged in an agreement PRIOR to the manufacture or supply of the product.
It is only copyright that creates the notion that payment can occur over a century after production and publication but before another member of the public is permitted to receive/view a copy of such a published work.
This is why a payment API is as fundamentally inappropriate to HTML as copyright is to the Internet.
The only thing you need is a means of (as securely as possible) identifying a work and its author and provenance. There is scope for standardising this in HTML.
Given a published work and details of its author and provenance there is ample information available to would be patrons of that author and those whose shoulders they’ve stood upon, e.g. “I wish to patronise this author’s future production to the tune of £5 per subsequent work in this series or class – until further notice”.