Sam asked for spec links for them: appearance: https://drafts.csswg.org/css-ui-4/#propdef-appearance aspect-ratio: https://drafts.csswg.org/css-sizing-4/#propdef-aspect-ratio backdrop-filter: https://drafts.fxtf.org/filter-effects-2/#propdef-backdrop-filter backface-visibility: https://drafts.csswg.org/css-transforms-2/#propdef-backface-visibility box-decoration-break: https://drafts.csswg.org/css-break-3/#propdef-box-decoration-break initial-letter: https://drafts.csswg.org/css-inline-3/#propdef-initial-letter line-clamp: https://drafts.csswg.org/css-overflow-3/#propdef-line-clamp mask: https://drafts.fxtf.org/css-masking-1/#propdef-mask mask-box-image: NONE! mask-clip: https://drafts.fxtf.org/css-masking-1/#propdef-mask-clip mask-composite: https://drafts.fxtf.org/css-masking-1/#propdef-mask-composite mask-image: https://drafts.fxtf.org/css-masking-1/#propdef-mask-image mask-origin: https://drafts.fxtf.org/css-masking-1/#propdef-mask-origin mask-position: https://drafts.fxtf.org/css-masking-1/#propdef-mask-position mask-position-x: NONE! mask-position-y: NONE! mask-repeat: https://drafts.fxtf.org/css-masking-1/#propdef-mask-repeat mask-size: https://drafts.fxtf.org/css-masking-1/#propdef-mask-size ruby-position: https://drafts.csswg.org/css-ruby-1/#propdef-ruby-position text-align-last: https://drafts.csswg.org/css-text-3/#propdef-text-align-last text-justify: https://drafts.csswg.org/css-text-3/#propdef-text-justify text-size-adjust: https://drafts.csswg.org/css-size-adjust-1/#propdef-text-size-adjust user-select: https://drafts.csswg.org/css-ui-4/#propdef-user-select https://drafts.csswg.org/css-ui/#propdef-user-select Also see standardised prefixed properties: -webkit-appearance: https://drafts.csswg.org/css-ui/#propdef--webkit-appearance https://drafts.csswg.org/css-ui-4/#propdef--webkit-appearance -webkit-aspect-ratio: NONE! -webkit-backdrop-filter: NONE! -webkit-backface-visibility: https://compat.spec.whatwg.org/#propdef--webkit-backface-visibility -webkit-box-decoration-break: NONE! -webkit-initial-letter: NONE! -webkit-line-clamp: https://drafts.csswg.org/css-overflow-3/#propdef--webkit-line-clamp -webkit-mask: https://compat.spec.whatwg.org/#propdef--webkit-mask -webkit-mask-box-image: https://compat.spec.whatwg.org/#propdef--webkit-mask-box-image -webkit-mask-clip: https://compat.spec.whatwg.org/#propdef--webkit-mask-clip -webkit-mask-composite: https://compat.spec.whatwg.org/#propdef--webkit-mask-composite -webkit-mask-image: https://compat.spec.whatwg.org/#propdef--webkit-mask-image -webkit-mask-origin: https://compat.spec.whatwg.org/#propdef--webkit-mask-origin -webkit-mask-position: https://compat.spec.whatwg.org/#propdef--webkit-mask-position -webkit-mask-position-x: NONE! -webkit-mask-position-y: NONE! -webkit-mask-repeat: https://compat.spec.whatwg.org/#propdef--webkit-mask-repeat -webkit-mask-size: https://compat.spec.whatwg.org/#propdef--webkit-mask-size -webkit-ruby-position: NONE! -webkit-text-align-last: NONE! -webkit-text-justify: NONE! -webkit-text-size-adjust: https://compat.spec.whatwg.org/#propdef--webkit-text-size-adjust -webkit-user-select: NONE!

Okay, so https://github.com/web-platform-tests/wpt/pull/26070 gets rid of the one instance of mask-box-image in WPT, and the references to mask-position-x/mask-position-y don't actually do anything, which leaves us only with properties where the unprefixed versions have current specs.

<rdar://problem/70384212>

So, 2023 update: Properties we don't support unprefixed which appear in WPT: 1 mask-position-x 1 mask-position-y 3 text-size-adjust 36 user-select 37 box-decoration-break 47 initial-letter 63 ruby-position 81 backdrop-filter I can't find any usage of any CSS value we only support prefixed.

Naïvely: ``` % % rg -U '(?-m)([\s{]|^)(font\-smoothing|locale|text\-size\-adjust|text\-zoom|logical\-height|border\-after|border\-after\-color|border\-after\-style|border\-after\-width|border\-before|border\-before\-color|border\-before\-style|border\-before\-width|border\-end|border\-end\-color|border\-end\-style|border\-end\-width|border\-start|border\-start\-color|border\-start\-style|border\-start\-width|cursor\-visibility|logical\-width|margin\-after|margin\-before|margin\-end|margin\-start|mask\-box\-image\-outset|mask\-box\-image\-repeat|mask\-box\-image\-slice|mask\-box\-image\-source|mask\-box\-image\-width|mask\-position\-x|mask\-position\-y|max\-logical\-height|max\-logical\-width|min\-logical\-height|min\-logical\-width|padding\-after|padding\-before|padding\-end|padding\-start|border\-horizontal\-spacing|border\-vertical\-spacing|box\-align|box\-direction|box\-flex|box\-flex\-group|box\-lines|box\-ordinal\-group|box\-orient|box\-pack|box\-reflect|column\-axis|column\-break\-after|column\-break\-before|column\-break\-inside|column\-progression|box\-decoration\-break|font\-size\-delta|hyphenate\-limit\-after|hyphenate\-limit\-before|hyphenate\-limit\-lines|initial\-letter|line\-box\-contain|line\-align|line\-grid|line\-snap|marquee\-direction|marquee\-increment|marquee\-repetition|marquee\-speed|marquee\-style|mask\-box\-image|mask\-source\-type|nbsp\-mode|rtl\-ordering|text\-combine|text\-decorations\-in\-effect|text\-fill\-color|text\-security|text\-stroke|text\-stroke\-color|text\-stroke\-width|user\-drag|user\-modify|user\-select|tap\-highlight\-color|overflow\-scrolling|touch\-callout)(\s+:|:)' --only-matching --replace='$2' --no-filename --glob '!third_party' | sort | uniq -c | sort -n 1 box-reflect 1 line-grid 1 mask-position-x 1 mask-position-y 1 padding-end 1 user-modify 5 locale 5 padding-start 10 text-size-adjust 59 user-select 64 initial-letter 91 box-decoration-break ``` To quickly triage: > box-reflect Unspecified, occurs only in html/rendering/the-details-element/details-autofocus-crash.html, unsupported by any browser. > line-grid Unspecified, occurs only in html/rendering/the-details-element/details-autofocus-crash.html, unsupported by any browser. > mask-position-x > mask-position-y See https://github.com/w3c/fxtf-drafts/issues/103. These only really appear in css/css-cascade/all-prop-revert-layer.html, which is probably not the most important case. > padding-end > padding-start Unspecified, these occur only in comments. > user-modify Unspecified. This occurs only in css/css-ui/tentative/user-modify-*, which are explicitly checking for non-support of the unprefixed form. > locale Unspecified. The matches are all false positives, mostly from JS. > text-size-adjust > user-select > initial-letter > box-decoration-break These four do actually meaningfully exist in WPT. And on the value side, based on we only have false positives, per: ``` rg -U '(?-m)(\:\s*|^\s*)(mini-control|small-control|control|body|pictograph|control-background|activelink|link|focus-ring-color|baseline-middle|box|fill-available|optimize-contrast|gradient|named-image|paged-x|paged-y)(\s*;|\s*}|\s*$)' --glob '!third_party' ```