I am currently working on getting the autocapitalize attribute standardized as part of the HTML spec:
It's mostly based on the current implementation in iOS Safari, so hopefully WebKit doesn't need to make any changes there. However, currently the attribute is not exposed on macOS Safari. We decided it should probably be exposed there for two reasons:
1. The IDL interface should be consistent across platforms, even where no input methods support autocapitalization.
2. Some macOS input methods may in fact want to allow web authors to control autocapitalization. For example, Chrome for Android respects the attribute for voice input, and macOS also supports voice input.