diff --git a/.phpcs.xml.dist b/.phpcs.xml.dist
index 05822e38c..b791b3496 100644
--- a/.phpcs.xml.dist
+++ b/.phpcs.xml.dist
@@ -13,6 +13,7 @@
 
     <!-- Include only the directories containing PHP files. -->
     <file>bin/</file>
+    <file>include/</file>
     <file>src/</file>
     <file>tests/</file>
 
diff --git a/composer.json b/composer.json
index ac0da0f6b..91747e556 100644
--- a/composer.json
+++ b/composer.json
@@ -49,7 +49,10 @@
   "autoload": {
     "psr-4": {
       "AmpProject\\": "src/"
-    }
+    },
+    "files": [
+      "include/compatibility-fixes.php"
+    ]
   },
   "autoload-dev": {
     "psr-4": {
diff --git a/include/compatibility-fixes.php b/include/compatibility-fixes.php
new file mode 100644
index 000000000..6cb69d683
--- /dev/null
+++ b/include/compatibility-fixes.php
@@ -0,0 +1,14 @@
+<?php
+
+namespace AmpProject;
+
+/**
+ * @var array<class-string<CompatibilityFix>> $compatibilityFixes
+ */
+$compatibilityFixes = [
+    CompatibilityFix\MovedClasses::class,
+];
+
+foreach ($compatibilityFixes as $compatibilityFix) {
+    $compatibilityFix::register();
+}
diff --git a/phpstan.neon.dist b/phpstan.neon.dist
index 028a78b2a..4c98e5f04 100644
--- a/phpstan.neon.dist
+++ b/phpstan.neon.dist
@@ -5,6 +5,7 @@ parameters:
 	level: 5
 	inferPrivatePropertyTypeFromConstructor: true
 	paths:
+		- include/
 		- src/
 	ignoreErrors:
 		- '#^Unsafe usage of new static\(\).$#'
diff --git a/src/CompatibilityFix.php b/src/CompatibilityFix.php
new file mode 100644
index 000000000..06b3198dd
--- /dev/null
+++ b/src/CompatibilityFix.php
@@ -0,0 +1,19 @@
+<?php
+
+namespace AmpProject;
+
+/**
+ * Compatibility fix that can be registered.
+ *
+ * @package ampproject/amp-toolbox
+ */
+interface CompatibilityFix
+{
+
+    /**
+     * Register the compatibility fix.
+     *
+     * @return void
+     */
+    public static function register();
+}
diff --git a/src/CompatibilityFix/MovedClasses.php b/src/CompatibilityFix/MovedClasses.php
new file mode 100644
index 000000000..7e01283ac
--- /dev/null
+++ b/src/CompatibilityFix/MovedClasses.php
@@ -0,0 +1,58 @@
+<?php
+
+namespace AmpProject\CompatibilityFix;
+
+use AmpProject\CompatibilityFix;
+
+/**
+ * Backwards compatibility fix for classes that were moved.
+ *
+ * @package ampproject/amp-toolbox
+ */
+final class MovedClasses implements CompatibilityFix
+{
+
+    /**
+     * Mapping of aliases to be registered.
+     *
+     * @var array<string, string> Associative array of class alias mappings.
+     */
+    const ALIASES = [
+        // v0.9.0 - moved HTML-based utility into a separate `Html` sub-namespace.
+        'AmpProject\AtRule'             => 'AmpProject\Html\AtRule',
+        'AmpProject\Attribute'          => 'AmpProject\Html\Attribute',
+        'AmpProject\LengthUnit'         => 'AmpProject\Html\LengthUnit',
+        'AmpProject\RequestDestination' => 'AmpProject\Html\RequestDestination',
+        'AmpProject\Role'               => 'AmpProject\Html\Role',
+        'AmpProject\Tag'                => 'AmpProject\Html\Tag',
+
+        // v0.9.0 - extracted `Encoding` out of `Dom\Document`, as it is turned into AMP value object.
+        'AmpProject\Dom\Document\Encoding' => 'AmpProject\Encoding',
+
+    ];
+
+    /**
+     * Register the compatibility fix.
+     *
+     * @return void
+     */
+    public static function register()
+    {
+        spl_autoload_register(__CLASS__ . '::autoloader');
+    }
+
+    /**
+     * Autoloader to register.
+     *
+     * @param string $oldClassName Old class name that was requested to be autoloaded.
+     * @return void
+     */
+    public static function autoloader($oldClassName)
+    {
+        if (! array_key_exists($oldClassName, self::ALIASES)) {
+            return;
+        }
+
+        class_alias(self::ALIASES[$oldClassName], $oldClassName, true);
+    }
+}
diff --git a/tests/CompatibilityFix/MovedClassesTest.php b/tests/CompatibilityFix/MovedClassesTest.php
new file mode 100644
index 000000000..20fed1fc7
--- /dev/null
+++ b/tests/CompatibilityFix/MovedClassesTest.php
@@ -0,0 +1,22 @@
+<?php
+
+namespace AmpProject\CompatibilityFix;
+
+use AmpProject\Tests\TestCase;
+
+/**
+ * Tests for AmpProject\CompatibilityFix\MovedClasses.
+ *
+ * @covers \AmpProject\CompatibilityFix\MovedClasses
+ * @package ampproject/amp-toolbox
+ */
+class MovedClassesTest extends TestCase
+{
+
+    public function testClassAliasesExist()
+    {
+        foreach (array_keys(MovedClasses::ALIASES) as $old) {
+            $this->assertTrue(class_exists($old) || interface_exists($old));
+        }
+    }
+}