https://a11yproject.com/ has patterns, a checklist for checking web accessibility, resources, and events.
awesome-a11y has a list of a number of great resources for developing accessible applications: https://github.com/brunopulis/awesome-a11y
In terms of W3C specifications [1], you've got: WAI-ARIA (Web Accessibility Initiative: Accessibile Rich Internet Applications) [2], and WCAG: Web Content Accessibility Guidelines [3]. The new W3C Payment Request API [4] makes it easy for browsers to offer a standard (and probably(?) already accessible) interface for the payment data entry screen, at least.
There are a number of automated accessibility testing platforms. "[W3C WAI] Web Accessibility Evaluation Tools List" [5] lists quite a few. Can someone recommend a good accessibility testing tools? Is Google Lighthouse (now included with Chrome Devtools and as a standalone script) a good tool for accessibility reviews?
[1] https://github.com/brunopulis/awesome-a11y/blob/master/topic...
[2] https://www.w3.org/TR/using-aria/
[3] https://www.w3.org/WAI/standards-guidelines/wcag/