I would like to know if accessibility settings in iOS such as the Button Shapes feature or other similar accessibility features can be applied to content within a webview.
For example, would buttons in a webview adapt to the Button Shapes setting when it is toggled on?
Check out CSS MQ Level 5: https://drafts.csswg.org/mediaqueries-5/
The prefers-reduced-motion and prefers-reduced-transparency features are 1-to-1 correlations with the native API features on Apple devices.
prefers-contrast has gone through iterations, so YMMV with regard to support, but the current syntax of @media (prefers-contrast: more) is intended to match the "Increase Contrast" feature on Apple devices, as well as similar higher contrast features on Windows and Android. Some of the values being discussed in that feature are Windows-specific or otherwise not relevant to iOS or macOS.
There is currently no Web API feature equivalent to the Button Shapes setting.
You can also use these media features with the window.matchMedia interface.