diff --git a/php-transformer/src/HtmlToBlocks/DocumentNormalizationPolicy.php b/php-transformer/src/HtmlToBlocks/DocumentNormalizationPolicy.php
new file mode 100644
index 00000000..111b9432
--- /dev/null
+++ b/php-transformer/src/HtmlToBlocks/DocumentNormalizationPolicy.php
@@ -0,0 +1,42 @@
+<?php
+declare(strict_types=1);
+
+namespace Automattic\BlocksEngine\PhpTransformer\HtmlToBlocks;
+
+/**
+ * Extracts the usable fallback from complete legacy frameset documents before
+ * DOM parsing can retain frame navigation as page content.
+ */
+final class DocumentNormalizationPolicy
+{
+    /**
+     * @return array{html: string, fallback: array<string, mixed>|null}
+     */
+    public function normalize(string $html): array
+    {
+        if (
+            ! preg_match('/<frameset\b[^>]*>/i', $html)
+            || ! preg_match('/<\/frameset\s*>/i', $html)
+            || ! preg_match('/<noframes\b[^>]*>(.*?)<\/noframes\s*>/is', $html, $matches)
+        ) {
+            return array(
+                'html'     => $html,
+                'fallback' => null,
+            );
+        }
+
+        $fallbackHtml = preg_replace('/<\/?frameset\b[^>]*>/i', '', $matches[1]) ?? $matches[1];
+        $fallbackHtml = preg_replace('/<frame\b[^>]*>/i', '', $fallbackHtml) ?? $fallbackHtml;
+
+        return array(
+            'html' => $fallbackHtml,
+            'fallback' => FallbackDiagnostic::build(array(
+                'type'            => 'legacy_frameset_navigation',
+                'reason'          => 'unsupported_legacy_frameset_navigation',
+                'diagnostic_code' => 'html_legacy_frameset_navigation',
+                'message'         => 'Legacy frameset navigation was removed and its noframes fallback content was converted.',
+                'tag'             => 'frameset',
+            )),
+        );
+    }
+}
diff --git a/php-transformer/src/HtmlToBlocks/FallbackDiagnostic.php b/php-transformer/src/HtmlToBlocks/FallbackDiagnostic.php
index 8813811b..fa773531 100644
--- a/php-transformer/src/HtmlToBlocks/FallbackDiagnostic.php
+++ b/php-transformer/src/HtmlToBlocks/FallbackDiagnostic.php
@@ -192,6 +192,19 @@ private static function defaults(array $fields): array
                 'suggested_primitive'   => 'core/html',
                 'materialization_hint'  => 'preserve_sanitized_markup_until_a_specific_block_mapping_exists',
             ),
+            'html_legacy_frameset_navigation' => array(
+                'severity'              => 'warning',
+                'conversion_classification' => 'unsupported_loss',
+                'loss_class'            => 'unsupported_legacy_navigation_loss',
+                'diagnostic_class'      => 'unsupported_legacy_navigation',
+                'preservation_strategy' => 'convert_noframes_fallback_content',
+                'runtime_requirement'   => 'none',
+                'recoverability'        => 'recoverable_with_manual_navigation_mapping',
+                'actionability'         => 'map_legacy_frame_destinations_to_pages_or_navigation_blocks',
+                'suggested_repair_class' => 'map_legacy_frameset_navigation',
+                'suggested_primitive'   => 'navigation',
+                'materialization_hint'  => 'convert_the_noframes_fallback_content_and_rebuild_frame_destinations_as_standard_navigation',
+            ),
             'interactive_control_behavior_lost' => array(
                 'severity'              => 'warning',
                 'conversion_classification' => 'behavior_loss',
diff --git a/php-transformer/src/HtmlToBlocks/HtmlTransformer.php b/php-transformer/src/HtmlToBlocks/HtmlTransformer.php
index baf3b01a..78397130 100644
--- a/php-transformer/src/HtmlToBlocks/HtmlTransformer.php
+++ b/php-transformer/src/HtmlToBlocks/HtmlTransformer.php
@@ -406,7 +406,8 @@ public function transform(string $html, array $options = array()): TransformerRe
             ), $this->fallbackProvenance),
         );
 
-        $normalizedHtml = $this->normalizeHtml5VoidElements($this->documentBodyHtml($html));
+        $documentNormalization = ( new DocumentNormalizationPolicy() )->normalize($html);
+        $normalizedHtml = $this->normalizeHtml5VoidElements($this->documentBodyHtml($documentNormalization['html']));
         $document = new DOMDocument();
         $previous = libxml_use_internal_errors(true);
         $loaded   = $document->loadHTML('<?xml encoding="utf-8" ?><body>' . $normalizedHtml . '</body>', LIBXML_HTML_NOIMPLIED | LIBXML_HTML_NODEFDTD);
@@ -461,7 +462,7 @@ public function transform(string $html, array $options = array()): TransformerRe
             );
         }
 
-        $fallbacks   = array();
+        $fallbacks   = null === $documentNormalization['fallback'] ? array() : array( $documentNormalization['fallback'] );
         $interactionCandidates = $this->interactionCandidates($body);
         $this->collectSupersededNavToggleSelectors($body);
         $blocks      = $this->deduplicateNavigationBlocks($this->convertChildren($body, $fallbacks, true));
diff --git a/php-transformer/tests/contract/finding-contract.php b/php-transformer/tests/contract/finding-contract.php
index 6255650a..aebc11ce 100644
--- a/php-transformer/tests/contract/finding-contract.php
+++ b/php-transformer/tests/contract/finding-contract.php
@@ -257,6 +257,27 @@
     $totalFindings += $walk($result, "html:{$name}");
 }
 
+$framesetResult = ( new HtmlTransformer() )->transform(
+    '<!doctype html><html><frameset cols="180,*"><frame src="#main-content" name="main"><noframes><body><main id="main-content"><p>Kept fallback</p></main></body></noframes></frameset></html>'
+)->toArray();
+$totalFindings += $walk($framesetResult, 'html:legacy-frameset');
+$assert(
+    'html_legacy_frameset_navigation' === ($framesetResult['fallbacks'][0]['diagnostic_code'] ?? null),
+    'legacy frameset navigation emits a typed diagnostic'
+);
+$assert(
+    str_contains((string) ($framesetResult['serialized_blocks'] ?? ''), 'Kept fallback'),
+    'legacy frameset conversion retains noframes fallback content'
+);
+$assert(
+    ! str_contains((string) ($framesetResult['serialized_blocks'] ?? ''), '<frame'),
+    'legacy frameset conversion excludes frame navigation markup'
+);
+$assert(
+    ! str_contains((string) ($framesetResult['serialized_blocks'] ?? ''), '#main-content'),
+    'legacy frameset conversion excludes frame source values'
+);
+
 // Canvas requires a runtime selector hint to be flagged as a runtime island.
 $canvasResult = ( new HtmlTransformer() )->transform(
     '<main><canvas id="scene">Fallback</canvas><script>document.getElementById("scene").getContext("2d");</script></main>',
diff --git a/php-transformer/tests/fixtures/parity/html-legacy-frameset-noframes-fallback.json b/php-transformer/tests/fixtures/parity/html-legacy-frameset-noframes-fallback.json
new file mode 100644
index 00000000..511aee97
--- /dev/null
+++ b/php-transformer/tests/fixtures/parity/html-legacy-frameset-noframes-fallback.json
@@ -0,0 +1,25 @@
+{
+  "schema": "blocks-engine/php-transformer/parity-fixture/v1",
+  "name": "html-legacy-frameset-noframes-fallback",
+  "description": "Converts noframes fallback content while removing unsupported self-referential legacy frame navigation.",
+  "operation": "html_transformer.transform",
+  "input": {
+    "content": "<!doctype html><html><head><title>Legacy frame page</title></head><frameset cols=\"180,*\"><frame src=\"#main-content\" name=\"main\"><noframes><body><main id=\"main-content\"><p>Kept fallback</p></main></body></noframes></frameset></html>"
+  },
+  "expected_blocks": [
+    { "path": "blocks.0", "name": "core/group" },
+    { "path": "blocks.0.innerBlocks.0", "name": "core/paragraph", "attrs": { "content": "Kept fallback" } }
+  ],
+  "expected_fallbacks": [
+    { "type": "legacy_frameset_navigation", "reason": "unsupported_legacy_frameset_navigation", "diagnostic_code": "html_legacy_frameset_navigation", "tag": "frameset" }
+  ],
+  "expect": [
+    { "path": "status", "assert": "equals", "value": "success" },
+    { "path": "fallbacks", "assert": "count", "count": 1 },
+    { "path": "diagnostics.1.code", "assert": "equals", "value": "html_legacy_frameset_navigation" },
+    { "path": "serialized_blocks", "assert": "contains", "value": "Kept fallback" },
+    { "path": "serialized_blocks", "assert": "not_contains", "value": "<frame" },
+    { "path": "serialized_blocks", "assert": "not_contains", "value": "<frameset" },
+    { "path": "serialized_blocks", "assert": "not_contains", "value": "#main-content" }
+  ]
+}