Add accessibility-checklist.md (#200)

Based on the [FAST checklist](https://w3c.github.io/apa/fast/checklist.html).

Author: ianbjacobs

accessibility-checklist.md

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+# FAST Checklist for Secure Payment Confirmation
+
+The following evaluation is based on the [FAST checklist](https://w3c.github.io/apa/fast/checklist.html).
+
+## If technology allows visual rendering of content
+
+SPC does not include any features that result in rendering content in a page.
+
+When Secure Payment Confirmation is called at authentication time, the browser displays the following information in a transaction dialog owned by the browser:
+
+* payeeName and/or payeeOrigin (both strings)
+* instrument displayName and icon (an image or link to an image). The specification discusses how to use the displayName as alt text for the icon.
+* Transaction total and currency (both strings).
+
+## If technology provides author control over color
+
+N/A
+
+## If technology provides features to accept user input
+
+N/A
+
+## If technology provides user interaction features
+
+* The transaction dialog includes browser-owned buttons to cancel or proceed with Web Authentication.
+* [Accessibility considerations for WebAuthn](https://www.w3.org/TR/webauthn/#sctn-accessiblility-considerations) are documented in that specification.
+
+## If technology defines document semantics
+
+N/A
+
+## If technology provides time-based visual media
+
+N/A
+
+## If technology provides audio
+
+N/A
+
+## If technology allows time limits
+
+* SPC relies on the timeout parameter of Web Authentication.
+* Relevant [accessibility considerations for WebAuthn](https://www.w3.org/TR/webauthn/#sctn-accessiblility-considerations) are documented in that specification.
+
+## If technology allows text content
+
+* The SPC transaction dialog includes both an icon and string to help the user identify the relevant payment instrument. 
+## If technology creates objects that don't have an inherent text representation
+
+N/A
+
+## If technology provides content fallback mechanisms, whether text or other formats
+
+N/A
+
+## If technology provides visual graphics
+
+N/A
+
+## If technology provides internationalization support
+
+N/A
+
+## If technology defines accessible alternative features
+
+N/A
+
+## If technology provides content directly for end-users
+
+N/A
+
+## If technology defines an API
+
+* SPC relies on the useragent to generate a user interface (the transaction dialog). Previous review of Secure Payment Confirmation concluded there was no need to review the specification ([issue 14](https://github.com/w3c/a11y-request/issues/14#issuecomment-915393934)) and the specification is largely the same since that review.
+
+
+## If technology defines a transmission protocol
+
+N/A