diff --git a/resources/idlharness.js b/resources/idlharness.js
index 10d96f6a4e34b62..f3a2f6704bb4906 100644
--- a/resources/idlharness.js
+++ b/resources/idlharness.js
@@ -177,27 +177,9 @@ self.IdlArray = function()
      * might contain a partial interface or includes statement that depends
      * on a later one.  Save these up and handle them right before we run
      * tests.
-     *
-     * .partials is simply an array of objects from WebIDLParser.js'
-     * "partialinterface" production.  .includes maps strings to arrays of
-     * strings, such that
-     *
-     *   A includes B;
-     *   A includes C;
-     *   D includes E;
-     *
-     * results in this["includes"] = { A: ["B", "C"], D: ["E"] }.
-     *
-     * Similarly,
-     *
-     *   interface A : B {};
-     *   interface B : C {};
-     *
-     * results in this["inheritance"] = { A: "B", B: "C" }
      */
     this.partials = [];
-    this["includes"] = {};
-    this["inheritance"] = {};
+    this.includes = [];
 
     /**
      * Record of skipped IDL items, in case we later realize that they are a
@@ -249,11 +231,15 @@ IdlArray.prototype.internal_add_dependency_idls = function(parsed_idls, options)
     const new_options = { only: [] }
 
     const all_deps = new Set();
-    Object.values(this.inheritance).forEach(v => all_deps.add(v));
-    // NOTE: If 'A includes B' for B that we care about, then A is also a dep.
-    Object.keys(this.includes).forEach(k => {
-        all_deps.add(k);
-        this.includes[k].forEach(v => all_deps.add(v));
+    Object.values(this.members).forEach(v => {
+        if (v.base) {
+            all_deps.add(v.base);
+        }
+    });
+    // Add both 'A' and 'B' for each 'A includes B' entry.
+    this.includes.forEach(i => {
+        all_deps.add(i.target);
+        all_deps.add(i.includes);
     });
     this.partials.forEach(p => all_deps.add(p.name));
     // Add 'TypeOfType' for each "typedef TypeOfType MyType;" entry.
@@ -434,11 +420,7 @@ IdlArray.prototype.internal_add_idls = function(parsed_idls, options)
             {
                 return;
             }
-            if (!(parsed_idl.target in this["includes"]))
-            {
-                this["includes"][parsed_idl.target] = [];
-            }
-            this["includes"][parsed_idl.target].push(parsed_idl["includes"]);
+            this.includes.push(parsed_idl);
             return;
         }
 
@@ -452,16 +434,6 @@ IdlArray.prototype.internal_add_idls = function(parsed_idls, options)
             throw new IdlHarnessError("Duplicate identifier " + parsed_idl.name);
         }
 
-        if (parsed_idl["inheritance"]) {
-            // NOTE: Clash should be impossible (would require redefinition of parsed_idl.name).
-            if (parsed_idl.name in this["inheritance"]
-                && parsed_idl["inheritance"] != this["inheritance"][parsed_idl.name]) {
-                throw new IdlHarnessError(
-                    `Inheritance for ${parsed_idl.name} was already defined`);
-            }
-            this["inheritance"][parsed_idl.name] = parsed_idl["inheritance"];
-        }
-
         switch(parsed_idl.type)
         {
         case "interface":
@@ -529,34 +501,6 @@ IdlArray.prototype.prevent_multiple_testing = function(name)
     this.members[name].prevent_multiple_testing = true;
 };
 
-IdlArray.prototype.recursively_get_includes = function(interface_name)
-{
-    /**
-     * Helper function for test().  Returns an array of things that implement
-     * interface_name, so if the IDL contains
-     *
-     *   A includes B;
-     *   B includes C;
-     *   B includes D;
-     *
-     * then recursively_get_includes("A") should return ["B", "C", "D"].
-     */
-    var ret = this["includes"][interface_name];
-    if (ret === undefined)
-    {
-        return [];
-    }
-    for (var i = 0; i < this["includes"][interface_name].length; i++)
-    {
-        ret = ret.concat(this.recursively_get_includes(ret[i]));
-        if (ret.indexOf(ret[i]) != ret.lastIndexOf(ret[i]))
-        {
-            throw new IdlHarnessError("Circular includes statements involving " + ret[i]);
-        }
-    }
-    return ret;
-};
-
 IdlArray.prototype.is_json_type = function(type)
 {
     /**
@@ -753,37 +697,8 @@ IdlArray.prototype.test = function()
     /** Entry point.  See documentation at beginning of file. */
 
     // First merge in all partial definitions and interface mixins.
-    this.collapse_partials();
-
-    for (var lhs in this["includes"])
-    {
-        this.recursively_get_includes(lhs).forEach(function(rhs)
-        {
-            var errStr = lhs + " includes " + rhs + ", but ";
-            if (!(lhs in this.members)) throw errStr + lhs + " is undefined.";
-            if (!(this.members[lhs] instanceof IdlInterface)) throw errStr + lhs + " is not an interface.";
-            if (!(rhs in this.members)) throw errStr + rhs + " is undefined.";
-            if (!(this.members[rhs] instanceof IdlInterface)) throw errStr + rhs + " is not an interface.";
-
-            if (this.members[rhs].members.length) {
-                test(function () {
-                    var clash = this.members[rhs].members.find(function(member) {
-                        return this.members[lhs].members.find(function(m) {
-                            return this.are_duplicate_members(m, member);
-                        }.bind(this));
-                    }.bind(this));
-                    this.members[rhs].members.forEach(function(member) {
-                        assert_true(
-                            this.members[lhs].members.every(m => !this.are_duplicate_members(m, member)),
-                            "member " + member.name + " is unique");
-                        this.members[lhs].members.push(new IdlInterfaceMember(member));
-                    }.bind(this));
-                    assert_true(!clash, "member " + (clash && clash.name) + " is unique");
-                }.bind(this), lhs + " includes " + rhs + ": member names are unique");
-            }
-        }.bind(this));
-    }
-    this["includes"] = {};
+    this.merge_partials();
+    this.merge_includes();
 
     // Assert B defined for A : B
     for (const member of Object.values(this.members).filter(m => m.base)) {
@@ -832,7 +747,7 @@ IdlArray.prototype.test = function()
     }
 };
 
-IdlArray.prototype.collapse_partials = function()
+IdlArray.prototype.merge_partials = function()
 {
     const testedPartials = new Map();
     this.partials.forEach(function(parsed_idl)
@@ -926,6 +841,39 @@ IdlArray.prototype.collapse_partials = function()
     this.partials = [];
 }
 
+IdlArray.prototype.merge_includes = function()
+{
+    for (const parsed_idl of this.includes)
+    {
+        const lhs = parsed_idl.target;
+        const rhs = parsed_idl.includes;
+
+        var errStr = lhs + " includes " + rhs + ", but ";
+        if (!(lhs in this.members)) throw errStr + lhs + " is undefined.";
+        if (!(this.members[lhs] instanceof IdlInterface)) throw errStr + lhs + " is not an interface.";
+        if (!(rhs in this.members)) throw errStr + rhs + " is undefined.";
+        if (!(this.members[rhs] instanceof IdlInterface)) throw errStr + rhs + " is not an interface.";
+
+        if (this.members[rhs].members.length) {
+            test(function () {
+                var clash = this.members[rhs].members.find(function(member) {
+                    return this.members[lhs].members.find(function(m) {
+                        return this.are_duplicate_members(m, member);
+                    }.bind(this));
+                }.bind(this));
+                this.members[rhs].members.forEach(function(member) {
+                    assert_true(
+                        this.members[lhs].members.every(m => !this.are_duplicate_members(m, member)),
+                        "member " + member.name + " is unique");
+                    this.members[lhs].members.push(new IdlInterfaceMember(member));
+                }.bind(this));
+                assert_true(!clash, "member " + (clash && clash.name) + " is unique");
+            }.bind(this), lhs + " includes " + rhs + ": member names are unique");
+        }
+    }
+    this.includes = [];
+}
+
 IdlArray.prototype.are_duplicate_members = function(m1, m2) {
     if (m1.name !== m2.name) {
         return false;
@@ -1416,7 +1364,7 @@ IdlInterface.prototype.get_inheritance_stack = function() {
  */
 IdlInterface.prototype.default_to_json_operation = function(callback) {
     var map = new Map(), isDefault = false;
-    this.traverse_inherited_and_consequential_interfaces(function(I) {
+    this.get_inheritance_stack().forEach(function(I) {
         if (I.has_default_to_json_regular_operation()) {
             isDefault = true;
             I.members.forEach(function(m) {
@@ -1431,56 +1379,6 @@ IdlInterface.prototype.default_to_json_operation = function(callback) {
     return isDefault ? map : null;
 };
 
-/**
- * Traverses inherited interfaces from the top down
- * and imeplemented interfaces inside out.
- * Invokes |callback| on each interface.
- *
- * This is an abstract implementation of the traversal
- * algorithm specified in:
- * https://heycam.github.io/webidl/#collect-attribute-values
- * Given the following inheritance tree:
- *
- *           F
- *           |
- *       C   E - I
- *       |   |
- *       B - D
- *       |
- *   G - A - H - J
- *
- * Invoking traverse_inherited_and_consequential_interfaces() on A
- * would traverse the tree in the following order:
- * C -> B -> F -> E -> I -> D -> A -> G -> H -> J
- */
-
-IdlInterface.prototype.traverse_inherited_and_consequential_interfaces = function(callback) {
-    if (typeof callback != "function") {
-        throw new TypeError();
-    }
-    var stack = this.get_inheritance_stack();
-    _traverse_inherited_and_consequential_interfaces(stack, callback);
-};
-
-function _traverse_inherited_and_consequential_interfaces(stack, callback) {
-    var I = stack.pop();
-    callback(I);
-    var mixins = I.array["includes"][I.name];
-    if (mixins) {
-        mixins.forEach(function(id) {
-            var mixin = I.array.members[id];
-            if (!mixin) {
-                throw new Error("Interface mixin " + id + " not found (included by " + I.name + ")");
-            }
-            var interfaces = mixin.get_inheritance_stack();
-            _traverse_inherited_and_consequential_interfaces(interfaces, callback);
-        });
-    }
-    if (stack.length > 0) {
-        _traverse_inherited_and_consequential_interfaces(stack, callback);
-    }
-}
-
 IdlInterface.prototype.test = function()
 {
     if (this.has_extended_attribute("LegacyNoInterfaceObject") || this.is_mixin())
diff --git a/resources/test/tests/functional/idlharness/IdlDictionary/test_partial_interface_of.html b/resources/test/tests/functional/idlharness/IdlDictionary/test_partial_interface_of.html
index 5ee3fe7e893f19e..05e6e0b1e06c634 100644
--- a/resources/test/tests/functional/idlharness/IdlDictionary/test_partial_interface_of.html
+++ b/resources/test/tests/functional/idlharness/IdlDictionary/test_partial_interface_of.html
@@ -41,7 +41,7 @@
       idlArray.add_idls(`
         partial dictionary C {};
         namespace C {};`);
-      idlArray.collapse_partials();
+      idlArray.merge_partials();
     })();
   </script>
   <script type="text/json" id="expected">
diff --git a/resources/test/tests/functional/idlharness/IdlInterface/test_interface_mixin.html b/resources/test/tests/functional/idlharness/IdlInterface/test_interface_mixin.html
index a9a9094fe30ab50..8e0fd318c1c5cf8 100644
--- a/resources/test/tests/functional/idlharness/IdlInterface/test_interface_mixin.html
+++ b/resources/test/tests/functional/idlharness/IdlInterface/test_interface_mixin.html
@@ -17,30 +17,56 @@
   <script>
     "use strict";
 
-    // No original existence
+    // Simple includes statement (valid)
     (() => {
       const idlArray = new IdlArray();
-      idlArray.add_idls('partial interface mixin A {};');
-      idlArray.test();
+      idlArray.add_idls(`
+        [Exposed=Window] interface I1 {};
+        interface mixin M1 { attribute any a1; };
+        I1 includes M1;`);
+      idlArray.merge_partials();
+      idlArray.merge_includes();
+    })();
+
+    // Partial interface mixin (valid)
+    (() => {
+      const idlArray = new IdlArray();
+      idlArray.add_idls(`
+        [Exposed=Window] interface I2 {};
+        interface mixin M2 {};
+        partial interface mixin M2 { attribute any a2; };
+        I2 includes M2;`);
+      idlArray.merge_partials();
+      idlArray.merge_includes();
+    })();
+
+    // Partial interface mixin without original mixin
+    (() => {
+      const idlArray = new IdlArray();
+      idlArray.add_idls('partial interface mixin M3 {};');
+      idlArray.merge_partials();
+      idlArray.merge_includes();
     })();
 
-    // Name clash (partials)
+    // Name clash between mixin and partial mixin
     (() => {
       const idlArray = new IdlArray();
       idlArray.add_idls(`
-        interface mixin B { attribute any F; };
-        partial interface mixin B { attribute any F; };`);
-      idlArray.collapse_partials();
+        interface mixin M3 { attribute any a3; };
+        partial interface mixin M3 { attribute any a3; };`);
+      idlArray.merge_partials();
+      idlArray.merge_includes();
     })();
 
-    // Name clash (different mixins)
+    // Name clash between interface and mixin
     (() => {
       const idlArray = new IdlArray();
       idlArray.add_untested_idls(`
-        interface mixin C { attribute any F; };
-        interface D { attribute any F; };
-        D includes C;`);
-      idlArray.test();
+        interface mixin M4 { attribute any a4; };
+        interface I4 { attribute any a4; };
+        I4 includes I4;`);
+      idlArray.merge_partials();
+      idlArray.merge_includes();
     })();
   </script>
   <script type="text/json" id="expected">
diff --git a/resources/test/tests/functional/idlharness/IdlInterface/test_partial_interface_of.html b/resources/test/tests/functional/idlharness/IdlInterface/test_partial_interface_of.html
index d0e83af966410c0..72b8ee1dafd58b0 100644
--- a/resources/test/tests/functional/idlharness/IdlInterface/test_partial_interface_of.html
+++ b/resources/test/tests/functional/idlharness/IdlInterface/test_partial_interface_of.html
@@ -39,7 +39,7 @@
 
         [Exposed=(Worker)]
         partial interface C {};`);
-      idlArray.collapse_partials();
+      idlArray.merge_partials();
     })();
 
     // Invalid exposure
@@ -51,7 +51,7 @@
       idlArray.add_idls(`
         [Exposed=(DedicatedWorker)]
         partial interface D {};`);
-      idlArray.collapse_partials();
+      idlArray.merge_partials();
     })();
 
     // Original is a namespace, not an interface.
@@ -60,7 +60,7 @@
       idlArray.add_idls(`
         partial interface E {};
         namespace E {};`);
-      idlArray.collapse_partials();
+      idlArray.merge_partials();
     })();
 
     (() => {
@@ -100,7 +100,7 @@
       idlArray.add_idls(`
         interface M { attribute any A; };
         partial interface M { attribute any A; };`);
-      idlArray.collapse_partials();
+      idlArray.merge_partials();
     })();
   </script>
   <script type="text/json" id="expected">
diff --git a/resources/test/tests/functional/idlharness/IdlNamespace/test_partial_namespace.html b/resources/test/tests/functional/idlharness/IdlNamespace/test_partial_namespace.html
index c327a8b001d27e5..c68adfa941773dd 100644
--- a/resources/test/tests/functional/idlharness/IdlNamespace/test_partial_namespace.html
+++ b/resources/test/tests/functional/idlharness/IdlNamespace/test_partial_namespace.html
@@ -40,7 +40,7 @@
 
         [Exposed=(Worker)]
         partial namespace C {};`);
-      idlArray.collapse_partials();
+      idlArray.merge_partials();
     })();
 
     // Invalid exposure
@@ -52,7 +52,7 @@
       idlArray.add_idls(`
         [Exposed=(DedicatedWorker)]
         partial namespace D {};`);
-      idlArray.collapse_partials();
+      idlArray.merge_partials();
     })();
 
     // Original is an interface, not a namespace.
@@ -61,7 +61,7 @@
       idlArray.add_idls(`
         partial namespace E {};
         interface E {};`);
-      idlArray.collapse_partials();
+      idlArray.merge_partials();
     })();
   </script>
   <script type="text/json" id="expected">