Update proposal based on feedback

Fixes #4, #5 and #6

README.md

@@ -1,4 +1,4 @@
-# Web Preference API (name TBD)
+# Web Preferences API (name TBD)
 
 Authors: [Luke Warlow](https://github.com/lukewarlow)
 
@@ -31,13 +31,12 @@ It is intended for this override to apply permanently and be scoped per origin.
 ### Goals
 
 - Provide a way for sites to override a given user preference in a way that fully integrates with existing browser APIs
-- Provide a way for sites to determine the user's preference for a given setting, without having to resort to `matchMedia`
 - Increase usage of these preferences, leading to a more accessible web
 
 ### Non-Goals
 
 - Provide a way for sites to store arbitrary site-specific preferences -- local storage or other storage APIs should be used instead
-- Provide a way for sites to determine the origin of a user preference, beyond User Agent VS site (e.g. a site won't be able to determinate if a setting comes from the OS or browser)
+- Provide a way for sites to determine the source of a user preference, beyond User Agent VS site (e.g. a site won't be able to determine if a setting comes from the OS or browser)
 - Force browsers to provide a UI for overriding OS level user preferences (although this would be nice)
 - Force browsers to provide a UI for overriding user preferences per site (although this would be nice)
 
@@ -59,7 +58,7 @@ Like with the previous use case, if a site wanted to sync a user's animation pre
 
 With the **Web Preference API**, this would no longer be the case and sites could use a simple sync function on page load to ensure the server and client preference matches.
 
-This would have the added effect of the site benefiting from any potential UA stylesheet to reduce animations for users who have indicated a preference for reduced motion.
+This would have the added effect of the site benefiting from any potential future UA stylesheet to reduce animations for users who have indicated a preference for reduced motion.
 
 ### Fully Themed Browser UI
 
@@ -71,26 +70,25 @@ With the **Web Preference API**, sites could simply use the `color-scheme` prope
 
 ## Proposed Solution
 
-### The `navigator.preference` object
+### The `navigator.preferences` object
 
-A new `navigator.preference` object will be added to the platform. This object will be the entry point to this API.
+A new `navigator.preferences` object will be added to the platform. This object will be the entry point to this API.
 
 ```ts
 interface Navigator {
   readonly preference: PreferenceManager;
 }
 
 interface PreferenceManager {
-  setOverride(name: string, value: string): Promise<void>;
-  clearOverride(name: string): Promise<void>;
-  get(name: string): Promise<PreferenceResult>;
-  getSupported(): Promise<PreferenceSupportData[]>;
-}
+  // null means the preference is not overriden
+  colorScheme: string | null; // "light" | "dark" | null
+  contrast: string | null; // "no-preference" | "more" | "less" | null
+  reducedMotion: string | null; // "no-preference" | "reduce" | null
+  reducedTransparency: string | null; // "no-preference" | "reduce" | null
+  reducedData: string | null; // "no-preference" | "reduce" | null
+  // Future preferences can be added here, the exact properties will be down to the browser support.
 
-interface PreferenceResult extends EventTarget {
-  readonly value: string;
-  readonly isOverride: boolean;
-  onchange: ((this: PreferenceResult, ev: Event) => any) | null;
+  getSupported(): Promise<PreferenceSupportData[]>;
 }
 
 interface PreferenceSupportData {
@@ -99,55 +97,44 @@ interface PreferenceSupportData {
 }
 ```
 
-### The `navigator.preference.setOverride` method
+### The `navigator.preferences.[preferenceName]` properties
+
+Each preference the browser supports will be exposed as a property on the `navigator.preferences` object.
 
-This method allows a site to override a given user preference. This method will:
-- Resolve when the preference has been successfully overridden.
-- Be rejected if the operation is not successful. It'll reject with a [`DOMException`](https://developer.mozilla.org/en-US/docs/Web/API/DOMException) value of:
-  - `NotSupportedError`: If the preference is not supported by the browser.
-  - `ValidationError`: If the provided value is not valid for the given preference.
-  - `OperationError`: If the operation failed for any other reason.
+Feature detection for a given preference is as simple as:
 
 ```js
-await navigator.preference.setOverride('prefers-contrast', 'more');
+const colorSchemeSupported = 'colorScheme' in navigator.preferences;
 ```
 
-### The `navigator.preference.clearOverride` method
-
-This method allows a site to clear an override for a given user preference. This method will:
-- Resolve when the preference has been successfully cleared.
-- Be rejected if the operation is not successful. It'll reject with a [`DOMException`](https://developer.mozilla.org/en-US/docs/Web/API/DOMException) value of:
-  - `NotSupportedError`: If the preference is not supported by the browser.
-  - `OperationError`: If the operation failed for any other reason.
+Each property will return null if not overridden or a string value indicating the preference.
 
 ```js
-await navigator.preference.clearOverride('prefers-contrast');
+const colorScheme = navigator.preferences.colorScheme; // "light" | "dark" | null
 ```
 
-### The `navigator.preference.get` method
+To clear an override and return the preference to the browser default, the property can be set to null.
+
+```js
+navigator.preferences.colorScheme = null;
+```
 
-This method allows a site to get the current value of a given user preference. This method will:
-- Resolve with an object containing the current value of the preference, along with whether this is a site override.
-- Be rejected if the operation is not successful. It'll reject with a [`DOMException`](https://developer.mozilla.org/en-US/docs/Web/API/DOMException) value of:
-  - `NotSupportedError`: If the preference is not supported by the browser.
-  - `OperationError`: If the operation failed for any other reason.
+To set a preference override, the property can be set to a valid value for the preference.
+If an invalid value is set then a [`DOMException`](https://developer.mozilla.org/en-US/docs/Web/API/DOMException) will be thrown with a `ValidationError` code.
 
 ```js
-const preferenceResult = await navigator.preference.get('prefers-contrast');
-console.log(preferenceResult.value); // 'more'
-console.log(preferenceResult.isOverride); // true
-preferenceResult.addEventListener('change', () => {
-  console.log(preferenceResult.value); // 'less'
-});
+navigator.preferences.colorScheme = 'dark';
 ```
 
-### The `navigator.preference.getSupported` method
+### The `navigator.preferences.getSupported` method
+
+This method allows a site to get the preferences supported by the browser. This is useful for sites that want to dynamically generate UI for overriding preferences.
 
-This method allows a site to get the preferences supported by the browser.
+It also allows sites to determine if a preference value is supported before attempting to set it.
 
 ```js
-const preferenceSupportData = await navigator.preference.getSupported();
-console.log(preferenceSupportData); // [ { name: 'prefers-contrast', values: ['more', 'less', 'no-preference'] }, ... ]
+const preferenceSupportData = await navigator.preferences.getSupported();
+console.log(preferenceSupportData); // [ { name: 'contrast', values: ['more', 'less', 'no-preference'] }, ... ]
 ```
 
 ## Privacy and Security Considerations
@@ -156,6 +143,16 @@ console.log(preferenceSupportData); // [ { name: 'prefers-contrast', values: ['m
 
 This API exposes no new fingerprinting surfaces beyond that which already exist in the platform.
 
+### Iframes
+
+TODO: Need to work out the exact specifics with regards to iframes both same-origin and cross-origin.
+
+Same-origin iframes should probably be updated with the parent frame's preference overrides but in an opaque manner.
+
+e.g. if the parent frame sets `colorScheme` to `dark` then the iframe should see `prefers-color-scheme` as dark but shouldn't read `navigator.preferences.colorScheme` as `dark`.
+
+Whereas, cross-origin iframes should probably not be updated with the parent frame's preference overrides. This is an unfortunate limitation but is probably necessary to prevent new forms of data exfiltration.
+
 ## Alternative Solutions
 
 ### Use a custom media query
@@ -176,17 +173,13 @@ This also doesn't fix the (relatively minor) issue of preference syncing across
 
 ## Open Questions
 
-- Do we need a clearOverride method? Could we just use setOverride with a value of `null` or `undefined`?
-- Do we need a clearAllOverrides method?
-- Do we need a way to get the preference value or is using `matchMedia` sufficient? (I think we need at least a getOverride method)
-- Do we need an API method to indicate the accepted values for a given preference?
-- Do we need a user activation requirement for set and clear? (e.g. clicking a button)
-  - This would remove the ability to automatically sync preferences.
-- Do we need a permission grant? Or at least integrate with permissions policy?
-- Do we need configuration for the scope of these overrides?
-- Do we need configuration for choosing session vs permanent override?
+- Do we need a way to get the "computed" preference value or is using `matchMedia` sufficient?
+  - The ergonomics of `matchMedia` particularly aren't great when you want to listen to updates.
+- Do we need a user activation requirement for setting the values? (e.g. clicking a button)
+- Do we need configuration for the scope of these overrides? (I think this is out of scope)
+- Do we need configuration for choosing session vs permanent override? (I think this is out of scope)
 
 ## Acknowledgements
 
-Special thanks to [Ryan Christian](https://github.com/rschristian) for his help in reviewing this explainer and providing feedback.
+Special thanks to [Ryan Christian](https://github.com/rschristian) for his help in reviewing the original explainer and providing feedback.