diff --git a/Libraries/LibWeb/CSS/Parser/RuleParsing.cpp b/Libraries/LibWeb/CSS/Parser/RuleParsing.cpp
index 8a56ac35b9e..8ebd1a1f94a 100644
--- a/Libraries/LibWeb/CSS/Parser/RuleParsing.cpp
+++ b/Libraries/LibWeb/CSS/Parser/RuleParsing.cpp
@@ -30,6 +30,8 @@
 #include <LibWeb/CSS/FontFace.h>
 #include <LibWeb/CSS/Parser/ErrorReporter.h>
 #include <LibWeb/CSS/Parser/Parser.h>
+#include <LibWeb/CSS/Parser/Syntax.h>
+#include <LibWeb/CSS/Parser/SyntaxParsing.h>
 #include <LibWeb/CSS/PropertyName.h>
 #include <LibWeb/CSS/StyleValues/CSSKeywordValue.h>
 #include <LibWeb/CSS/StyleValues/CustomIdentStyleValue.h>
@@ -747,12 +749,40 @@ GC::Ptr<CSSPropertyRule> Parser::convert_to_property_rule(AtRule const& rule)
         }
     });
 
-    // TODO: Parse the initial value using the syntax, if it's provided.
-
-    if (syntax_maybe.has_value() && inherits_maybe.has_value()) {
-        return CSSPropertyRule::create(realm(), name, syntax_maybe.value(), inherits_maybe.value(), move(initial_value_maybe));
+    // @property rules require a syntax and inherits descriptor; if either are missing, the entire rule is invalid and must be ignored.
+    if (!syntax_maybe.has_value() || syntax_maybe->is_empty() || !inherits_maybe.has_value()) {
+        return {};
     }
-    return {};
+
+    auto parsing_params = CSS::Parser::ParsingParams { *document() };
+    auto syntax_component_values = parse_component_values_list(parsing_params, syntax_maybe.value());
+    auto maybe_syntax = parse_as_syntax(syntax_component_values);
+
+    // If the provided string is not a valid syntax string (if it returns failure when consume
+    // a syntax definition is called on it), the descriptor is invalid and must be ignored.
+    if (!maybe_syntax) {
+        return {};
+    }
+    // The initial-value descriptor is optional only if the syntax is the universal syntax definition,
+    // otherwise the descriptor is required; if it’s missing, the entire rule is invalid and must be ignored.
+    if (!initial_value_maybe && maybe_syntax->type() != CSS::Parser::SyntaxNode::NodeType::Universal) {
+        return {};
+    }
+
+    if (initial_value_maybe) {
+        initial_value_maybe = Web::CSS::Parser::parse_with_a_syntax(parsing_params, initial_value_maybe->tokenize(), *maybe_syntax);
+        // Otherwise, if the value of the syntax descriptor is not the universal syntax definition,
+        // the following conditions must be met for the @property rule to be valid:
+        //  - The initial-value descriptor must be present.
+        //  - The initial-value descriptor’s value must parse successfully according to the grammar specified by the syntax definition.
+        //  - FIXME: The initial-value must be computationally independent.
+
+        if (!initial_value_maybe || initial_value_maybe->is_guaranteed_invalid()) {
+            return {};
+        }
+    }
+
+    return CSSPropertyRule::create(realm(), name, syntax_maybe.value(), inherits_maybe.value(), move(initial_value_maybe));
 }
 
 GC::Ptr<CSSFontFaceRule> Parser::convert_to_font_face_rule(AtRule const& rule)
diff --git a/Tests/LibWeb/Text/expected/wpt-import/css/css-properties-values-api/at-property-cssom.txt b/Tests/LibWeb/Text/expected/wpt-import/css/css-properties-values-api/at-property-cssom.txt
new file mode 100644
index 00000000000..904e1801ea0
--- /dev/null
+++ b/Tests/LibWeb/Text/expected/wpt-import/css/css-properties-values-api/at-property-cssom.txt
@@ -0,0 +1,44 @@
+Harness status: OK
+
+Found 39 tests
+
+39 Pass
+Pass	Rule for --no-descriptors is invalid
+Pass	Rule for --no-syntax is invalid
+Pass	Rule for --no-inherits is invalid
+Pass	Rule for --no-initial-color-value is invalid
+Pass	Rule for --syntax-only is invalid
+Pass	Rule for --inherits-only is invalid
+Pass	Rule for --initial-value-only is invalid
+Pass	Rule for --valid has expected cssText
+Pass	Rule for --valid-reverse has expected cssText
+Pass	Rule for --valid-universal has expected cssText
+Pass	Rule for --valid-whitespace has expected cssText
+Pass	Rule for --vALId has expected cssText
+Pass	Rule for --no-initial-universal-value has expected cssText
+Pass	Rule for --tab	tab has expected cssText
+Pass	CSSRule.type returns 0
+Pass	Rule for --valid returns expected value for CSSPropertyRule.name
+Pass	Rule for --valid-reverse returns expected value for CSSPropertyRule.name
+Pass	Rule for --valid-universal returns expected value for CSSPropertyRule.name
+Pass	Rule for --valid-whitespace returns expected value for CSSPropertyRule.name
+Pass	Rule for --vALId returns expected value for CSSPropertyRule.name
+Pass	Rule for --no-initial-universal-value returns expected value for CSSPropertyRule.name
+Pass	Rule for --valid returns expected value for CSSPropertyRule.syntax
+Pass	Rule for --valid-reverse returns expected value for CSSPropertyRule.syntax
+Pass	Rule for --valid-universal returns expected value for CSSPropertyRule.syntax
+Pass	Rule for --valid-whitespace returns expected value for CSSPropertyRule.syntax
+Pass	Rule for --vALId returns expected value for CSSPropertyRule.syntax
+Pass	Rule for --no-initial-universal-value returns expected value for CSSPropertyRule.syntax
+Pass	Rule for --valid returns expected value for CSSPropertyRule.inherits
+Pass	Rule for --valid-reverse returns expected value for CSSPropertyRule.inherits
+Pass	Rule for --valid-universal returns expected value for CSSPropertyRule.inherits
+Pass	Rule for --valid-whitespace returns expected value for CSSPropertyRule.inherits
+Pass	Rule for --vALId returns expected value for CSSPropertyRule.inherits
+Pass	Rule for --no-initial-universal-value returns expected value for CSSPropertyRule.inherits
+Pass	Rule for --valid returns expected value for CSSPropertyRule.initialValue
+Pass	Rule for --valid-reverse returns expected value for CSSPropertyRule.initialValue
+Pass	Rule for --valid-universal returns expected value for CSSPropertyRule.initialValue
+Pass	Rule for --valid-whitespace returns expected value for CSSPropertyRule.initialValue
+Pass	Rule for --vALId returns expected value for CSSPropertyRule.initialValue
+Pass	Rule for --no-initial-universal-value returns expected value for CSSPropertyRule.initialValue
\ No newline at end of file
diff --git a/Tests/LibWeb/Text/expected/wpt-import/css/css-values/attr-all-types.txt b/Tests/LibWeb/Text/expected/wpt-import/css/css-values/attr-all-types.txt
index 9ad09a7bc4f..537678a1486 100644
--- a/Tests/LibWeb/Text/expected/wpt-import/css/css-values/attr-all-types.txt
+++ b/Tests/LibWeb/Text/expected/wpt-import/css/css-values/attr-all-types.txt
@@ -2,8 +2,8 @@ Harness status: OK
 
 Found 139 tests
 
-130 Pass
-9 Fail
+128 Pass
+11 Fail
 Pass	CSS Values and Units Test: attr
 Fail	CSS Values and Units Test: attr 1
 Pass	CSS Values and Units Test: attr 2
@@ -54,12 +54,12 @@ Pass	CSS Values and Units Test: attr 46
 Pass	CSS Values and Units Test: attr 47
 Pass	CSS Values and Units Test: attr 48
 Pass	CSS Values and Units Test: attr 49
-Pass	CSS Values and Units Test: attr 50
+Fail	CSS Values and Units Test: attr 50
 Fail	CSS Values and Units Test: attr 51
 Pass	CSS Values and Units Test: attr 52
 Pass	CSS Values and Units Test: attr 53
 Pass	CSS Values and Units Test: attr 54
-Pass	CSS Values and Units Test: attr 55
+Fail	CSS Values and Units Test: attr 55
 Pass	CSS Values and Units Test: attr 56
 Pass	CSS Values and Units Test: attr 57
 Pass	CSS Values and Units Test: attr 58
diff --git a/Tests/LibWeb/Text/expected/wpt-import/css/css-variables/variable-css-wide-keywords.txt b/Tests/LibWeb/Text/expected/wpt-import/css/css-variables/variable-css-wide-keywords.txt
index 7d6b52b0698..ba07e6608b9 100644
--- a/Tests/LibWeb/Text/expected/wpt-import/css/css-variables/variable-css-wide-keywords.txt
+++ b/Tests/LibWeb/Text/expected/wpt-import/css/css-variables/variable-css-wide-keywords.txt
@@ -2,15 +2,15 @@ Harness status: OK
 
 Found 30 tests
 
-11 Pass
-19 Fail
+9 Pass
+21 Fail
 Pass	`initial` as a value for an unregistered custom property
 Pass	`inherit` as a value for an unregistered custom property
 Pass	`unset` as a value for an unregistered custom property
 Fail	`revert` as a value for an unregistered custom property
 Pass	`revert-layer` as a value for an unregistered custom property
-Pass	`initial` as a value for a non-inheriting registered custom property
-Pass	`initial` as a value for an inheriting registered custom property
+Fail	`initial` as a value for a non-inheriting registered custom property
+Fail	`initial` as a value for an inheriting registered custom property
 Pass	`inherit` as a value for a non-inheriting registered custom property
 Pass	`inherit` as a value for an inheriting registered custom property
 Fail	`unset` as a value for a non-inheriting registered custom property
diff --git a/Tests/LibWeb/Text/input/wpt-import/css/css-properties-values-api/at-property-cssom.html b/Tests/LibWeb/Text/input/wpt-import/css/css-properties-values-api/at-property-cssom.html
new file mode 100644
index 00000000000..56d4ea7a27a
--- /dev/null
+++ b/Tests/LibWeb/Text/input/wpt-import/css/css-properties-values-api/at-property-cssom.html
@@ -0,0 +1,191 @@
+<!DOCTYPE html>
+<link rel="help" href="https://drafts.css-houdini.org/css-properties-values-api-1/#cssom">
+<script src="../../resources/testharness.js"></script>
+<script src="../../resources/testharnessreport.js"></script>
+<style>
+  @property --valid {
+    syntax: "<color> | none";
+    inherits: false;
+    initial-value: red;
+  }
+  @property --valid-reverse {
+    initial-value: 0px;
+    inherits: true;
+    syntax: "<length>";
+  }
+  @property --valid-universal {
+    syntax: "*";
+    inherits: false;
+  }
+  @property --valid-whitespace {
+    syntax: " <color># ";
+    inherits: false;
+    initial-value: red, blue;
+  }
+  @property --vALId {
+    syntax: "<color> | none";
+    inherits: false;
+    initial-value: red;
+  }
+  @property --no-descriptors {
+
+  }
+  @property --no-syntax {
+    inherits: false;
+    initial-value: red;
+  }
+  @property --no-inherits {
+    syntax: "<color> | none";
+    initial-value: red;
+  }
+  @property --no-initial-color-value {
+    syntax: "<color> | none";
+    inherits: false;
+  }
+  @property --no-initial-universal-value {
+    syntax: "*";
+    inherits: false;
+  }
+  @property --syntax-only {
+    syntax: "<color> | none";
+  }
+  @property --inherits-only {
+    inherits: true;
+  }
+  @property --initial-value-only {
+    initial-value: red;
+  }
+  /* U+0009 CHARACTER TABULATION */
+  @property --tab\9 tab {
+    syntax: "*";
+    inherits: true;
+  }
+</style>
+<script>
+
+function find_at_property_rule(name) {
+  for (let rule of document.styleSheets[0].cssRules) {
+    if (rule.constructor.name != "CSSPropertyRule")
+      continue;
+    if (rule.name == name)
+      return rule;
+  }
+  return null;
+}
+
+function test_invalid(name) {
+  test(() => {
+    let rule = find_at_property_rule(name);
+    assert_true(!rule);
+  }, `Rule for ${name} is invalid`);
+}
+
+function test_css_text(name, expected) {
+  test(() => {
+    let rule = find_at_property_rule(name);
+    assert_true(!!rule);
+    assert_equals(rule.cssText, expected);
+  }, `Rule for ${name} has expected cssText`);
+}
+
+function test_name(name) {
+  test(() => {
+    let rule = find_at_property_rule(name);
+    assert_true(!!rule);
+    assert_equals(rule.name, name);
+  }, `Rule for ${name} returns expected value for CSSPropertyRule.name`);
+}
+
+function test_syntax(name, expected) {
+  test(() => {
+    let rule = find_at_property_rule(name);
+    assert_true(!!rule);
+    assert_equals(rule.syntax, expected);
+  }, `Rule for ${name} returns expected value for CSSPropertyRule.syntax`);
+}
+
+function test_inherits(name, expected) {
+  test(() => {
+    let rule = find_at_property_rule(name);
+    assert_true(!!rule);
+    assert_equals(rule.inherits, expected);
+  }, `Rule for ${name} returns expected value for CSSPropertyRule.inherits`);
+}
+
+function test_initial_value(name, expected) {
+  test(() => {
+    let rule = find_at_property_rule(name);
+    assert_true(!!rule);
+    assert_equals(rule.initialValue, expected);
+  }, `Rule for ${name} returns expected value for CSSPropertyRule.initialValue`);
+}
+
+// Invalid @property rules.
+test_invalid('--no-descriptors');
+test_invalid('--no-syntax');
+test_invalid('--no-inherits');
+test_invalid('--no-initial-color-value');
+test_invalid('--syntax-only', '@property --syntax-only { syntax: "<color> | none"; }');
+test_invalid('--inherits-only', '@property --inherits-only { inherits: true; }');
+test_invalid('--initial-value-only', '@property --initial-value-only { initial-value: red; }');
+
+// CSSPropertyRule.cssText
+
+test_css_text('--valid', '@property --valid { syntax: "<color> | none"; inherits: false; initial-value: red; }');
+test_css_text('--valid-reverse', '@property --valid-reverse { syntax: "<length>"; inherits: true; initial-value: 0px; }');
+test_css_text('--valid-universal', '@property --valid-universal { syntax: "*"; inherits: false; }');
+test_css_text('--valid-whitespace', '@property --valid-whitespace { syntax: " <color># "; inherits: false; initial-value: red, blue; }');
+test_css_text('--vALId', '@property --vALId { syntax: "<color> | none"; inherits: false; initial-value: red; }');
+
+test_css_text('--no-initial-universal-value', '@property --no-initial-universal-value { syntax: "*"; inherits: false; }');
+
+test_css_text('--tab\ttab', '@property --tab\\9 tab { syntax: "*"; inherits: true; }');
+
+// CSSRule.type
+
+test(() => {
+  let rule = find_at_property_rule('--valid');
+  assert_equals(rule.type, 0);
+}, 'CSSRule.type returns 0');
+
+// CSSPropertyRule.name
+
+test_name('--valid');
+test_name('--valid-reverse');
+test_name('--valid-universal');
+test_name('--valid-whitespace');
+test_name('--vALId');
+
+test_name('--no-initial-universal-value');
+
+// CSSPropertyRule.syntax
+
+test_syntax('--valid', '<color> | none');
+test_syntax('--valid-reverse', '<length>');
+test_syntax('--valid-universal', '*');
+test_syntax('--valid-whitespace', ' <color># ');
+test_syntax('--vALId', '<color> | none');
+
+test_syntax('--no-initial-universal-value', '*');
+
+// CSSPropertyRule.inherits
+
+test_inherits('--valid', false);
+test_inherits('--valid-reverse', true);
+test_inherits('--valid-universal', false);
+test_inherits('--valid-whitespace', false);
+test_inherits('--vALId', false);
+
+test_inherits('--no-initial-universal-value', false);
+
+// CSSPropertyRule.initialValue
+
+test_initial_value('--valid', 'red');
+test_initial_value('--valid-reverse', '0px');
+test_initial_value('--valid-universal', null);
+test_initial_value('--valid-whitespace', 'red, blue');
+test_initial_value('--vALId', 'red');
+
+test_initial_value('--no-initial-universal-value', null);
+
+</script>