Adds attr features (#2109)

Co-authored-by: Patrick Brosset <[email protected]>
Co-authored-by: Daniel D. Beck <[email protected]>

Author: 3 people

features/attr-contents.yml

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+name: attr() (content only)
+description: "The `attr()` CSS function, in `content` property declarations, sets a `::before` or `::after` pseudo-element's content to the value of the specified HTML attribute."
+spec: https://drafts.csswg.org/css-values-5/#attr-notation
+group: css
+compat_features:
+  - css.types.attr

features/attr-contents.yml.dist

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+# Generated from: attr-contents.yml
+# Do not edit this file by hand. Edit the source file instead!
+
+status:
+  baseline: high
+  baseline_low_date: 2015-07-29
+  baseline_high_date: 2018-01-29
+  support:
+    chrome: "2"
+    chrome_android: "18"
+    edge: "12"
+    firefox: "1"
+    firefox_android: "4"
+    safari: "3.1"
+    safari_ios: "2"
+compat_features:
+  - css.types.attr

features/attr.yml

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+name: attr()
+description: "The `attr()` CSS function, in the context of any property, returns the value of an attribute of an HTML element, with the option to return it as a specific type or unit. You can set a default value for missing or invalid attributes."
+spec: https://drafts.csswg.org/css-values-5/#attr-notation
+caniuse: css3-attr
+group: css
+compat_features:
+  # The css.types.attr key makes sense here but is already used in `contents-attr`
+  #- css.types.attr
+  - css.types.attr.fallback
+  - css.types.attr.type-or-unit
+  - css.types.attr.type-or-unit.angle
+  - css.types.attr.type-or-unit.color
+  - css.types.attr.type-or-unit.frequency
+  - css.types.attr.type-or-unit.integer
+  - css.types.attr.type-or-unit.length
+  - css.types.attr.type-or-unit.number
+  - css.types.attr.type-or-unit.percentage
+  - css.types.attr.type-or-unit.time
+  - css.types.attr.type-or-unit.url

features/attr.yml.dist

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+# Generated from: attr.yml
+# Do not edit this file by hand. Edit the source file instead!
+
+status:
+  baseline: false
+  support: {}
+compat_features:
+  # baseline: false
+  # support:
+  #   firefox: "119"
+  #   firefox_android: "119"
+  - css.types.attr.fallback
+
+  # ⬇️ Same status as overall feature ⬇️
+  # baseline: false
+  # support: {}
+  - css.types.attr.type-or-unit
+  - css.types.attr.type-or-unit.angle
+  - css.types.attr.type-or-unit.color
+  - css.types.attr.type-or-unit.frequency
+  - css.types.attr.type-or-unit.integer
+  - css.types.attr.type-or-unit.length
+  - css.types.attr.type-or-unit.number
+  - css.types.attr.type-or-unit.percentage
+  - css.types.attr.type-or-unit.time
+  - css.types.attr.type-or-unit.url