diff --git a/src/browserlib/extract-cssdfn.mjs b/src/browserlib/extract-cssdfn.mjs
index 7b02da08..e61aa1a6 100644
--- a/src/browserlib/extract-cssdfn.mjs
+++ b/src/browserlib/extract-cssdfn.mjs
@@ -184,16 +184,25 @@ export default function () {
       }
       if (matchingValues.length === 0) {
         // Dangling production rule. That should never happen for properties,
-        // at-rules, descriptors and functions, since they should always be
-        // defined somewhere. That happens from time to time for types that are
-        // described in prose and that don't have a dfn. One could perhaps argue
-        // that these constructs ought to have a dfn too.
-        if (!res.warnings) {
-          res.warnings = []
+        // at-rules, descriptors: they should always be defined somewhere. That
+        // happens from time to time for functions and types that are defined
+        // in a spec and (temporarily) extended in another spec.
+        if (rule.name.match(/^<.*>$/)) {
+          const isFunction = !!rule.name.match(/\(\)/);
+          res.values.push({
+            name: isFunction ? rule.name.replace(/^<(.*)>$/, '$1') : rule.name,
+            type: isFunction ? 'function' : 'type',
+            value: rule.value
+          });
+        }
+        else {
+          if (!res.warnings) {
+            res.warnings = [];
+          }
+          const warning = Object.assign({ msg: 'Missing definition' }, rule);
+          warnings.push(warning);
+          rootDfns.push(warning);
         }
-        const warning = Object.assign({ msg: 'Missing definition' }, rule);
-        warnings.push(warning);
-        rootDfns.push(warning);
       }
     }
   }
diff --git a/test/extract-css.js b/test/extract-css.js
index 8a9b76b5..5d399af6 100644
--- a/test/extract-css.js
+++ b/test/extract-css.js
@@ -1010,16 +1010,29 @@ that spans multiple lines */
     ]
   },
 
+  {
+    title: 'creates a definition when one is missing for a type',
+    html: `
+    <pre class="prod">&lt;my-type> = none | auto</pre>
+    `,
+    propertyName: 'values',
+    css: [{
+      name: '<my-type>',
+      type: 'type',
+      value: 'none | auto'
+    }]
+  },
+
   {
     title: 'issues a warning when a definition is missing',
     html: `
-    <pre class="prod">&lt;my-type&gt; = none | auto
+    <pre class="prod">foo = bar</pre>
     `,
     propertyName: 'warnings',
     css: [{
       msg: 'Missing definition',
-      name: '<my-type>',
-      value: 'none | auto'
+      name: 'foo',
+      value: 'bar'
     }]
   },