Add holding features for unspecified platform features

features/unspecified-apis.yml

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+name: Unspecified APIs
+description: Some browsers implement unspecified APIs. This is an artificial feature that contains references to browser compatibility data entries for those additions.
+spec: link to our own documentation explaining ourselves in more detail
+discouraged:
+  according_to:
+    - TODO link to our own documentation explaining ourselves in more detail
+compat_features:
+  # Chrome implemented these APIs while they were in a PR for the spec, which
+  # was subsequently not included in the ultimately merged changes.
+  # https://github.com/w3c/payment-handler/pull/170#issuecomment-483453062
+  # https://issues.chromium.org/issues/40527525
+  - api.AbortPaymentEvent
+  - api.AbortPaymentEvent.AbortPaymentEvent
+  - api.AbortPaymentEvent.respondWith
+
+  # These `Screen` properties are broadly interoperable but unspecified.
+  # https://github.com/w3c/window-management/pull/76#discussion_r780596068
+  # contains the only discussion I could find of this fact.
+  - api.Screen.availLeft
+  - api.Screen.availLeft.relative-multi-screen
+  - api.Screen.availTop
+  - api.Screen.availTop.relative-multi-screen
+
+  # Dropped from Accessibility Object Model spec and potentially never shipped
+  # anywhere. https://wicg.github.io/aom/spec/phase2.html#privacy-concerns
+  # https://github.com/mdn/browser-compat-data/pull/25860
+  - api.Permissions.permission_accessibility-events

features/unspecified-css.yml

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+name: Unspecified CSS
+description: Some browsers implement unspecified CSS properties or values. This is an artificial feature that contains references to browser compatibility data entries for those additions.
+spec: TODO link to our own documentation explaining ourselves in more detail
+discouraged:
+  according_to:
+    - TODO link to our own documentation explaining ourselves in more detail
+compat_features:
+  # Removed from the platform, but not yet removed from BCD
+  - -moz-image-region # To be removed in March 2025

features/unspecified-html.yml

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+name: Unspecified HTML
+description: Some browsers implement unspecified HTML elements and attributes. This is an artificial feature that contains references to browser compatibility data entries for those additions.
+spec: TODO link to our own documentation explaining ourselves in more detail
+discouraged:
+  according_to:
+    - TODO link to our own documentation explaining ourselves in more detail
+compat_features:
+  # Stuck in WHATWG PR limbo:
+  # https://github.com/mdn/browser-compat-data/pull/25863
+  - api.HTMLAnchorElement.hrefTranslate
+  - html.elements.a.hreftranslate

features/unspecified-javascript.yml

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+name: Unspecified JavaScript
+description: Some browsers implement unspecified JavaScript properties or methods. This is an artificial feature that contains references to browser compatibility data entries for those additions.
+spec: link to our own documentation explaining ourselves in more detail
+discouraged:
+  according_to:
+    - TODO link to our own documentation explaining ourselves in more detail
+compat_features:
+  # Firefox-only Function property.
+  # There exists some discussion of getting rid
+  # of it: https://github.com/tc39/proposal-error-stacks/issues/14
+  - javascript.builtins.Function.displayName
+
+  # Firefox-only InternalError object
+  # Thrown on "too much recursion" while Safari and Chrome throw RangeError
+  # https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Too_much_recursion
+  - javascript.builtins.InternalError
+  - javascript.builtins.InternalError.InternalError
+
+  # Firefox-only Erorr properties
+  - javascript.builtins.Error.Error.lineNumber_parameter
+  - javascript.builtins.Error.lineNumber
+  - javascript.builtins.Error.Error.fileName_parameter
+  - javascript.builtins.Error.fileName