diff --git a/core/modules/filter/tests/src/Unit/FilterHtmlTest.php b/core/modules/filter/tests/src/Unit/FilterHtmlTest.php
index 53b583c..9196f17 100644
--- a/core/modules/filter/tests/src/Unit/FilterHtmlTest.php
+++ b/core/modules/filter/tests/src/Unit/FilterHtmlTest.php
@@ -173,4 +173,24 @@ public function stringArrayConversionTestCases() {
     ];
   }
 
+  /**
+   * Test the legacy allowed_html format works when using the FilterHtml plugin.
+   */
+  public function testLegacyAllowedHtmlFormat() {
+    $legacy_format = '<h1> <h2> <a href>';
+    $structured_format = [
+      'h1' => [],
+      'h2' => [],
+      'a' => [
+        'href' => '*',
+      ],
+    ];
+    // Test the legacy format is accepted when constructing the plugin directly.
+    $filter = new FilterHtml(['settings' => ['allowed_html' => $legacy_format]], 'filter_html', ['provider' => 'test']);
+    $this->assertSame($structured_format, $filter->getConfiguration()['settings']['allowed_html']);
+    // Test the legacy format is accepted when setting the configuration.
+    $filter->setConfiguration(['settings' => ['allowed_html' => $legacy_format]]);
+    $this->assertSame($structured_format, $filter->getConfiguration()['settings']['allowed_html']);
+  }
+
 }