@saschanaz this is a great PR and much needed cruft cleanup, and cross checking against collector results to make sure we're not deleting entries that actually have been implemented is a great idea. It's a pretty big PR to review/land at once though, could it be split into smaller bits? Did you use a script to produce it, so that it could be recreated on master at any time?

@ddbeck thoughts on this, given that you marked it not ready? What are the checks you'd like to see, other than careful manual review of each removal?

As a compat overview, here are the entries removed by this PR:

api.AuthenticatorAttestationResponse.getTransports
api.BluetoothAdvertisingData
api.BluetoothDevice.adData
api.BluetoothDevice.connectGATT
api.BluetoothDevice.deviceClass
api.BluetoothDevice.gattServer
api.BluetoothDevice.paired
api.BluetoothDevice.productID
api.BluetoothDevice.productVersion
api.BluetoothDevice.unwatchAdvertisements
api.BluetoothDevice.uuids
api.BluetoothDevice.vendorID
api.BluetoothDevice.vendorIDSource
api.BluetoothDevice.watchAdvertisements
api.BluetoothDevice.watchingAdvertisements
api.BluetoothRemoteGATTService.getCharacteristics
api.BluetoothRemoteGATTService.getIncludedService
api.BluetoothRemoteGATTService.isPrimary
api.BudgetService.getBudget
api.BudgetService.getCost
api.BudgetState
api.CSS.ic
api.CSS.lh
api.CSS.rlh
api.CSS.vb
api.CSS.vi
api.CanvasRenderingContext2D.addHitRegion.other_hit_region_options
api.CanvasRenderingContext2D.currentTransform.DOMMatrix_return_value
api.ConstrainULong
api.DOMConfiguration
api.DOMImplementationList
api.Document.dragexit_event
api.Document.mozSyntheticDocument
api.DocumentFragment.methods
api.DocumentOrShadowRoot.nodeFromPoint
api.DocumentOrShadowRoot.nodesFromPoint
api.HTMLButtonElement.menu
api.HTMLIFrameElement.setNfcFocus
api.HTMLIsIndexElement
api.HTMLMenuItemElement.command
api.HTMLTableElement.sortable
api.HTMLTableElement.stopSorting
api.IDBVersionChangeRequest.setVersion
api.NDEFMessage
api.NDEFReader
api.NDEFReadingEvent
api.NDEFRecord
api.NDEFWriter
api.PaymentAddress.regionCode
api.PerformanceFrameTiming
api.RTCIceTransport.gatheringstatechange_event
api.RTCIceTransport.getLocalCandidates
api.RTCIceTransport.getLocalParameters
api.RTCIceTransport.ongatheringstatechange
api.RTCIceTransport.onselectedcandidatepairchange
api.RTCIceTransport.onstatechange
api.RTCIceTransport.selectedcandidatepairchange_event
api.RTCIceTransport.statechange_event
api.RTCRemoteOutboundRtpStreamStats.reportsSent
api.RTCStatsReport
api.Range.compareNode
api.ReadableByteStreamController
api.ReadableStreamBYOBReader
api.ReadableStreamBYOBRequest
api.SVGMeshElement
api.ULongRange
api.Window.clipboardchange_event
api.Window.getAttention
api.Window.getAttentionWithCycleCount
api.Window.mozAnimationStartTime
api.Window.vrdisplayblur_event
api.XRPermissionDescriptor
api.XRPermissionStatus
css.at-rules.media.prefers-reduced-transparency
css.at-rules.media.scan
css.at-rules.media.scripting
css.at-rules.media.update
css.at-rules.page.bleed
css.at-rules.page.marks
css.at-rules.page.page-margin-boxes
css.at-rules.page.size
css.at-rules.viewport.viewport-fit
css.properties.azimuth
css.properties.break-after.multicol_context.always
css.properties.break-after.paged_context.recto
css.properties.break-after.region_context
css.properties.break-before.multicol_context.always
css.properties.break-before.paged_context.recto
css.properties.break-before.region_context
css.properties.break-inside.region_context
css.properties.column-fill.balance-all
css.properties.column-width.intrinsic_sizes
css.properties.font-variant.greek_accented_characters
css.properties.font-variant.uppercase_eszett
css.properties.initial-letter-align
css.properties.margin-trim
css.properties.mask-origin.fill-box
css.properties.mask-origin.stroke-box
css.properties.mask-origin.view-box
css.properties.offset-position
css.properties.ruby-position.inter-character
css.properties.text-align.justify-all
css.properties.text-align.string
css.properties.text-indent.each-line
css.properties.text-indent.hanging
css.properties.text-overflow.fade_function
css.properties.text-overflow.fade_value
css.selectors.blank
css.selectors.column
css.selectors.empty.matches_whitespace
css.selectors.first-letter.dutch_ij_digraph
css.selectors.grammar-error
css.selectors.has
css.selectors.spelling-error
css.types.attr.fallback
css.types.attr.type-or-unit
css.types.frequency-percentage
css.types.frequency
css.types.image.image
css.types.length-percentage.cap
css.types.length-percentage.ic
css.types.length-percentage.lh
css.types.length-percentage.rlh
css.types.length-percentage.vb
css.types.length-percentage.vi
css.types.length.cap
css.types.length.ic
css.types.length.lh
css.types.length.rlh
css.types.length.vb
css.types.length.vi
html.elements.command
html.elements.dir
html.elements.iframe.allowpaymentrequest
html.elements.isindex
html.elements.link.sizes
html.elements.listing
html.elements.menu.button_menus
html.elements.menu.type.type_toolbar
html.elements.multicol
html.elements.nextid
html.elements.textarea.autocapitalize
http.headers.Cache-Control.stale-if-error
http.headers.csp.Content-Security-Policy.navigate-to
http.headers.csp.Content-Security-Policy.prefetch-src
http.headers.Cookie2
http.headers.Feature-Policy.battery
http.headers.Feature-Policy.layout-animations
http.headers.Feature-Policy.picture-in-picture
http.headers.Feature-Policy.vibrate
http.headers.Feature-Policy.wake-lock
http.headers.Set-Cookie2
javascript.builtins.AsyncIterator.AsyncIterator
javascript.builtins.AsyncIterator.asIndexedPairs
javascript.builtins.AsyncIterator.drop
javascript.builtins.AsyncIterator.every
javascript.builtins.AsyncIterator.filter
javascript.builtins.AsyncIterator.find
javascript.builtins.AsyncIterator.flatMap
javascript.builtins.AsyncIterator.forEach
javascript.builtins.AsyncIterator.from
javascript.builtins.AsyncIterator.map
javascript.builtins.AsyncIterator.reduce
javascript.builtins.AsyncIterator.some
javascript.builtins.AsyncIterator.take
javascript.builtins.AsyncIterator.toArray
javascript.builtins.AsyncIterator.@@toStringTag
javascript.builtins.Iterator.Iterator
javascript.builtins.Iterator.asIndexedPairs
javascript.builtins.Iterator.drop
javascript.builtins.Iterator.every
javascript.builtins.Iterator.filter
javascript.builtins.Iterator.find
javascript.builtins.Iterator.flatMap
javascript.builtins.Iterator.forEach
javascript.builtins.Iterator.from
javascript.builtins.Iterator.map
javascript.builtins.Iterator.reduce
javascript.builtins.Iterator.some
javascript.builtins.Iterator.take
javascript.builtins.Iterator.toArray
javascript.builtins.Iterator.@@toStringTag
javascript.operators.pipeline
mathml.elements.math.overflow
mathml.elements.mglyph
mathml.elements.mlabeledtr
mathml.elements.mo.form
mathml.elements.mo.largeop
mathml.elements.mspace.linebreak
mathml.elements.mstyle.decimalpoint
mathml.elements.mstyle.infixbreakstyle
mathml.elements.mtable.alignmentscope
mathml.elements.mtable.columnwidth
mathml.elements.mtable.equalcolumns
mathml.elements.mtable.equalrows
mathml.elements.mtable.groupalign
mathml.elements.mtable.minlabelspacing
mathml.elements.mtable.side
mathml.elements.mtd.groupalign
mathml.elements.mtr.groupalign
svg.elements.textPath.method
webextensions.manifest.content_security_policy.isolated_world
webextensions.manifest.content_security_policy.sandbox
webextensions.manifest.manifest_version.v3
webextensions.match_patterns.scheme.ftps

Did you use a script to produce it, so that it could be recreated on master at any time?

Yes, this is from a customized version of #6809 where things are autodeleted. I'll try splitting this into smaller pieces.

Sorry, I apologize for forgetting to say why I marked it as not ready. I was going to suggest that we put it into draft status, like a few other PRs, and use it as a burndown for removing these topic-by-topic. For all-false features, I'm just not confident we can mass delete them. I think we need to do a little bit of work to make sure that there really was no implementation.

Alternatively, if we want to qualify each of these features within a big PR like this, I'm thinking we ought to hold back on this until the @mdn migration is done and then treat this as a semver minor version event.

Oh, also, there's a bit of work to do on MDN: if there exist actual pages for any of these, we should archive them. That's another thing I wanted to check before merging any of this.

I think review will inevitably turn up some issues that will take some back and forth, so splitting into a few groups would be great. I'd be happy to review smaller PRs and do some source code archeology for Blink, Gecko and WebKit to ensure no implementation has existed.

Thanks for the feedback. First things first, converting this to draft:

This pull request has merge conflicts that must be resolved before it can be merged.

This pull request has merge conflicts that must be resolved before it can be merged.

As a compat overview, here are the entries removed by this PR:

http.headers.csp.Content-Security-Policy.navigate-to
http.headers.csp.Content-Security-Policy.prefetch-src
http.headers.Cookie2 http.headers.Feature-Policy.battery
http.headers.Feature-Policy.layout-animations
http.headers.Feature-Policy.picture-in-picture
http.headers.Feature-Policy.vibrate
http.headers.Feature-Policy.wake-lock

1. I cannot find any data at the moment for the navigate-to, layout-animations or vibrate directives.
2. prefetch-src, battery, picture-in-picture and screen-wake-lock directives seem to be supported at least by some browsers (though as Permissions-Policy). I would therefore not remove them, but rather Firefox should implement support for them too.

This pull request has merge conflicts that must be resolved before it can be merged.