diff --git a/schemas/postprocessing/css.json b/schemas/postprocessing/css.json
index 00857fa8..ea071f17 100644
--- a/schemas/postprocessing/css.json
+++ b/schemas/postprocessing/css.json
@@ -9,6 +9,13 @@
         "type": "string"
       },
       "minItems": 1
+    },
+
+    "extended": {
+      "type": "array",
+      "items": {
+        "$ref": "../common.json#/$defs/url"
+      }
     }
   },
 
@@ -25,6 +32,7 @@
         "properties": {
           "name": { "type": "string", "pattern": "^@" },
           "href": { "$ref": "../common.json#/$defs/url" },
+          "extended": { "$ref": "#/$defs/extended" },
           "syntax": { "$ref": "../common.json#/$defs/cssValue" },
           "prose": { "type": "string" },
           "descriptors": {
@@ -54,6 +62,7 @@
           "name": { "type": "string", "pattern": "^.*()$" },
           "for": { "$ref": "#/$defs/scopes" },
           "href": { "$ref": "../common.json#/$defs/url" },
+          "extended": { "$ref": "#/$defs/extended" },
           "prose": { "type": "string" },
           "syntax": { "$ref": "../common.json#/$defs/cssValue" }
         }
@@ -68,6 +77,7 @@
         "properties": {
           "name": { "$ref": "../common.json#/$defs/cssPropertyName" },
           "href": { "$ref": "../common.json#/$defs/url" },
+          "extended": { "$ref": "#/$defs/extended" },
           "syntax": { "$ref": "../common.json#/$defs/cssValue" },
           "legacyAliasOf": { "$ref": "../common.json#/$defs/cssPropertyName" },
           "styleDeclaration": {
@@ -87,6 +97,7 @@
         "properties": {
           "name": { "$ref": "../common.json#/$defs/cssPropertyName" },
           "href": { "$ref": "../common.json#/$defs/url" },
+          "extended": { "$ref": "#/$defs/extended" },
           "prose": { "type": "string" },
           "syntax": { "$ref": "../common.json#/$defs/cssValue" }
         }
@@ -102,6 +113,7 @@
           "name": { "type": "string", "pattern": "^[a-zA-Z0-9-\\+\\(\\)\\[\\]\\{\\}]+$" },
           "for": { "$ref": "#/$defs/scopes" },
           "href": { "$ref": "../common.json#/$defs/url" },
+          "extended": { "$ref": "#/$defs/extended" },
           "prose": { "type": "string" },
           "syntax": { "$ref": "../common.json#/$defs/cssValue" }
         }
diff --git a/src/postprocessing/cssmerge.js b/src/postprocessing/cssmerge.js
index af199322..f939b643 100644
--- a/src/postprocessing/cssmerge.js
+++ b/src/postprocessing/cssmerge.js
@@ -111,8 +111,9 @@ export default {
 
     // The job is "almost" done but we now need to de-duplicate entries.
     // Duplicated entries exist when:
-    // - A property is defined in one spec and extended in other specs. We'll
-    // consolidate the entries (and syntaxes) to get back to a single entry.
+    // - A property, function or type is defined in one spec and extended in
+    // other specs. We'll consolidate the entries (and syntaxes) to get back to
+    // a single entry.
     // - An at-rule is defined in one spec. Additional descriptors are defined
     // in other specs. We'll consolidate the entries similarly.
     // - A descriptor is defined in one level of a spec series, and re-defined
@@ -179,13 +180,22 @@ export default {
       // Identify the base definition for each feature, using the definition
       // (that has some known syntax) in the most recent level. Move that base
       // definition to the beginning of the array and get rid of other base
-      // definitions.
-      // (Note: the code chooses one definition if duplicates of base
-      // definitions in unrelated specs still exist)
+      // definitions. Notes:
+      // - For properties, an extension is an entry with a `newValues` key.
+      // - For functions and types, an extension is an entry without an `href`
+      // key (the spec that extends the base type does not contain any `<dfn>`)
+      // - The code chooses one definition if duplicates of base definitions in
+      // unrelated specs still exist.
       for (const [name, dfns] of Object.entries(featureDfns)) {
-        let actualDfns = dfns.filter(dfn => dfn.syntax);
+        let actualDfns = dfns.filter(dfn => dfn.href && dfn.syntax);
         if (actualDfns.length === 0) {
-          actualDfns = dfns.filter(dfn => !dfn.newValues);
+          actualDfns = dfns.filter(dfn => dfn.href && !dfn.newValues);
+        }
+        if (actualDfns.length === 0) {
+          // No base definition, not a real type, let's discard it
+          // (problem should be captured through tests on individual extracts)
+          delete featureDfns[name];
+          continue;
         }
         const best = actualDfns.reduce((dfn1, dfn2) => {
           if (dfn1.spec.series.shortname !== dfn2.spec.series.shortname) {
@@ -199,6 +209,7 @@ export default {
             return dfn1;
           }
         });
+        best.extended = [];
         featureDfns[name] = [best].concat(
           dfns.filter(dfn => !actualDfns.includes(dfn))
         );
@@ -239,6 +250,18 @@ export default {
               continue;
             }
             baseDfn.syntax += ' | ' + dfn.newValues;
+            baseDfn.extended.push(dfn.href);
+          }
+          else if (dfn.syntax) {
+            // Extensions of functions and types are *re-definitions* in
+            // practice, new syntax overrides the base one. There should be
+            // only one such extension in unrelated specs, the code assumes
+            // that some sort of curation already took place, and picks up
+            // a winner randomly.
+            // Note: we don't have any `href` info for functions/types
+            // extensions, so we'll just use the URL of the crawled spec.
+            baseDfn.syntax = dfn.syntax;
+            baseDfn.extended = [dfn.spec.crawled ?? dfn.spec.url];
           }
           if (baseDfn.descriptors && dfn.descriptors?.length > 0) {
             baseDfn.descriptors.push(...dfn.descriptors.filter(desc =>
diff --git a/test/merge-css.js b/test/merge-css.js
index 1c8aca53..a79ae207 100644
--- a/test/merge-css.js
+++ b/test/merge-css.js
@@ -54,7 +54,7 @@ const descriptorBase = {
   type: 'discrete'
 };
 
-const descriptorExtended = Object.assign({}, descriptorBase, {
+const descriptorExtension = Object.assign({}, descriptorBase, {
   href: 'https://drafts.csswg.org/css-stuff-2/#descdef-descriptor',
   value: 'extended'
 });
@@ -77,6 +77,7 @@ const property1 = {
 
 const propertyLegacy = {
   name: 'good-old-overlay',
+  href: 'https://compat.spec.whatwg.org/#good-old-overlay',
   legacyAliasOf: 'overlay'
 };
 
@@ -93,6 +94,12 @@ const type1 = {
   value: 'repeat | space | round | no-repeat'
 };
 
+const type1Extension = {
+  name: '<repetition>',
+  type: 'type',
+  value: 'bis repetita'
+};
+
 const functionVar = {
   name: 'var()',
   href: 'https://drafts.csswg.org/css-variables-2/#funcdef-var',
@@ -113,14 +120,17 @@ const functionEnv = {
  * the outputs to the inputs directly. This conversion function takes
  * some object or value and converts it to ease comparisons.
  */
-function conv(entry) {
+function conv(entry, parentKey) {
   const res = {};
   if (typeof entry !== 'object') {
     return entry;
   }
+  if (entry.href && !entry.extended && parentKey !== 'descriptors') {
+    entry.extended = [];
+  }
   for (const [key, value] of Object.entries(entry)) {
     if (Array.isArray(value)) {
-      res[key] = value.map(conv);
+      res[key] = value.map(v => conv(v, key));
     }
     else if (key === 'value') {
       res.syntax = value;
@@ -309,7 +319,8 @@ describe('CSS extracts consolidation', function () {
           properties: [
             Object.assign({}, property1, {
               value: null,
-              newValues: 'train'
+              newValues: 'train',
+              href: 'https://drafts.csswg.org/css-otherstuff-2/#tchou-tchou'
             })
           ]
         })
@@ -318,7 +329,8 @@ describe('CSS extracts consolidation', function () {
     const result = await cssmerge.run({ results });
     assert.deepEqual(result.properties, [
       Object.assign({}, conv(property1), {
-        syntax: 'none | auto | train'
+        syntax: 'none | auto | train',
+        extended: ['https://drafts.csswg.org/css-otherstuff-2/#tchou-tchou']
       })
     ]);
   });
@@ -344,7 +356,8 @@ describe('CSS extracts consolidation', function () {
           properties: [
             Object.assign({}, property1, {
               value: null,
-              newValues: 'train'
+              newValues: 'train',
+              href: 'https://drafts.csswg.org/css-otherstuff-1/#tchou-tchou'
             })
           ]
         })
@@ -357,7 +370,8 @@ describe('CSS extracts consolidation', function () {
           properties: [
             Object.assign({}, property1, {
               value: null,
-              newValues: 'train'
+              newValues: 'train',
+              href: 'https://drafts.csswg.org/css-otherstuff-2/#tchou-tchou'
             })
           ]
         })
@@ -366,7 +380,8 @@ describe('CSS extracts consolidation', function () {
     const result = await cssmerge.run({ results });
     assert.deepEqual(result.properties, [
       Object.assign({}, conv(property1), {
-        syntax: 'none | auto | train'
+        syntax: 'none | auto | train',
+        extended: ['https://drafts.csswg.org/css-otherstuff-2/#tchou-tchou']
       })
     ]);
   });
@@ -432,7 +447,7 @@ describe('CSS extracts consolidation', function () {
         css: Object.assign({}, emptyExtract, {
           atrules: [
             Object.assign({}, atrule2, {
-              descriptors: [descriptorExtended]
+              descriptors: [descriptorExtension]
             })
           ]
         })
@@ -442,7 +457,7 @@ describe('CSS extracts consolidation', function () {
     assert.deepEqual(result.atrules, [
       conv(Object.assign({}, atrule2, {
         syntax: '@media foo',
-        descriptors: [descriptorExtended]
+        descriptors: [descriptorExtension]
       }))
     ]);
   });
@@ -630,4 +645,57 @@ describe('CSS extracts consolidation', function () {
       ]
     })));
   });
+
+  it('merges extended types', async () => {
+    const results = structuredClone([
+      {
+        shortname: 'css-stuff-1',
+        series: { shortname: 'css-stuff' },
+        seriesVersion: '1',
+        crawled: 'https://drafts.csswg.org/css-stuff-1/',
+        css: Object.assign({}, emptyExtract, {
+          values: [
+            Object.assign({}, type1)
+          ]
+        })
+      },
+      {
+        shortname: 'css-otherstuff-1',
+        series: { shortname: 'css-otherstuff' },
+        seriesVersion: '1',
+        crawled: 'https://drafts.csswg.org/css-otherstuff-1/',
+        css: Object.assign({}, emptyExtract, {
+          values: [
+            Object.assign({}, type1Extension)
+          ]
+        })
+      },
+    ]);
+    const result = await cssmerge.run({ results });
+    assert.deepEqual(result, conv(Object.assign({}, emptyMerged, {
+      types: [
+        Object.assign({}, conv(type1), {
+          syntax: type1Extension.value,
+          extended: ['https://drafts.csswg.org/css-otherstuff-1/']
+        })
+      ]
+    })));
+  });
+
+  it('discards type extensions without a base definition', async () => {
+    const results = structuredClone([
+      {
+        shortname: 'css-stuff-1',
+        series: { shortname: 'css-stuff' },
+        seriesVersion: '1',
+        css: Object.assign({}, emptyExtract, {
+          values: [
+            Object.assign({}, type1Extension)
+          ]
+        })
+      }
+    ]);
+    const result = await cssmerge.run({ results });
+    assert.deepEqual(result, conv(emptyMerged));
+  });
 });