diff --git a/packages/playground/data-liberation/src/block-markup/WP_HTML_To_Blocks.php b/packages/playground/data-liberation/src/block-markup/WP_HTML_To_Blocks.php
index 5e82314ca9..d4f2118e85 100644
--- a/packages/playground/data-liberation/src/block-markup/WP_HTML_To_Blocks.php
+++ b/packages/playground/data-liberation/src/block-markup/WP_HTML_To_Blocks.php
@@ -28,14 +28,15 @@ class WP_HTML_To_Blocks implements WP_Block_Markup_Converter {
 
 	private $state       = self::STATE_READY;
 	private $block_stack = array();
-	private $html;
+	private $markup_processor;
 	private $ignore_text            = false;
 	private $in_ephemeral_paragraph = false;
 	private $block_markup           = '';
 	private $metadata               = array();
+	private $last_error             = null;
 
-	public function __construct( $html ) {
-		$this->html = new \WP_HTML_Processor( $html );
+	public function __construct( $markup_processor ) {
+		$this->markup_processor = $markup_processor;
 	}
 
 	public function convert() {
@@ -43,21 +44,29 @@ public function convert() {
 			return false;
 		}
 
-		while ( $this->html->next_token() ) {
-			switch ( $this->html->get_token_type() ) {
+		while ( $this->markup_processor->next_token() ) {
+			var_dump( $this->markup_processor->get_token_type() );
+			switch ( $this->markup_processor->get_token_type() ) {
 				case '#text':
 					if ( $this->ignore_text ) {
 						break;
 					}
-					$this->append_rich_text( htmlspecialchars( $this->html->get_modifiable_text() ) );
+					$this->append_rich_text( htmlspecialchars( $this->markup_processor->get_modifiable_text() ) );
 					break;
 				case '#tag':
 					$this->handle_tag();
 					break;
 			}
 		}
+		var_dump( $this->markup_processor->get_last_error() );
+
+		if ( $this->markup_processor->get_last_error() ) {
+			$this->last_error = $this->markup_processor->get_last_error();
+			return false;
+		}
 
 		$this->close_ephemeral_paragraph();
+
 		return true;
 	}
 
@@ -77,8 +86,8 @@ public function get_block_markup() {
 	}
 
 	private function handle_tag() {
-		$html          = $this->html;
-		$tag           = $html->get_tag();
+		$html          = $this->markup_processor;
+		$tag           = strtoupper( $html->get_tag() );
 		$tag_lowercase = strtolower( $tag );
 
 		$is_tag_opener = ! $html->is_tag_closer();
@@ -304,7 +313,7 @@ private function should_preserve_tag_in_rich_text( $tag ) {
 	}
 
 	private function is_at_inline_code_element() {
-		$breadcrumbs = $this->html->get_breadcrumbs();
+		$breadcrumbs = $this->markup_processor->get_breadcrumbs();
 		foreach ( $breadcrumbs as $tag ) {
 			switch ( $tag ) {
 				case 'A':
@@ -392,4 +401,8 @@ private function close_ephemeral_paragraph() {
 			$this->in_ephemeral_paragraph = false;
 		}
 	}
+
+	public function get_last_error() {
+		return $this->last_error;
+	}
 }
diff --git a/packages/playground/data-liberation/src/entity-readers/WP_EPub_Entity_Reader.php b/packages/playground/data-liberation/src/entity-readers/WP_EPub_Entity_Reader.php
index d4bfb35320..e3a6c2a06b 100644
--- a/packages/playground/data-liberation/src/entity-readers/WP_EPub_Entity_Reader.php
+++ b/packages/playground/data-liberation/src/entity-readers/WP_EPub_Entity_Reader.php
@@ -26,14 +26,17 @@ class WP_EPub_Entity_Reader extends WP_Entity_Reader {
 	protected $current_post_id;
 	protected $remaining_html_files;
 	protected $current_html_reader;
-
+	protected $last_error;
 	public function __construct( WP_Zip_Filesystem $zip, $first_post_id = 1 ) {
 		$this->zip             = $zip;
 		$this->current_post_id = $first_post_id;
 	}
 
 	public function next_entity() {
-		// If we're finished, we're finished.
+		if ( $this->last_error ) {
+			return false;
+		}
+
 		if ( $this->finished ) {
 			return false;
 		}
@@ -92,16 +95,14 @@ public function next_entity() {
 
 			$html_file = array_shift( $this->remaining_html_files );
 			$html      = $this->zip->read_file( $html_file );
-			/**
-			 * @TODO: Don't just assume that WP_HTML_Entity_Reader can
-			 *        handle an XHTML file. We might run into XML-specific
-			 *        subtleties that will derail the process.
-			 *        Let's consider using WP_XML_Processor instead.
-			 */
-			$this->current_html_reader = new \WP_HTML_Entity_Reader(
-				$html,
+			$this->current_html_reader = new WP_HTML_Entity_Reader(
+				WP_XML_Processor::create_from_string( $html ),
 				$this->current_post_id
 			);
+			if ( $this->current_html_reader->get_last_error() ) {
+				$this->last_error = $this->current_html_reader->get_last_error();
+				return false;
+			}
 			++$this->current_post_id;
 		}
 
@@ -117,6 +118,6 @@ public function is_finished(): bool {
 	}
 
 	public function get_last_error(): ?string {
-		return null;
+		return $this->last_error;
 	}
 }
diff --git a/packages/playground/data-liberation/src/entity-readers/WP_HTML_Entity_Reader.php b/packages/playground/data-liberation/src/entity-readers/WP_HTML_Entity_Reader.php
index b01bd0c875..67ceadad7d 100644
--- a/packages/playground/data-liberation/src/entity-readers/WP_HTML_Entity_Reader.php
+++ b/packages/playground/data-liberation/src/entity-readers/WP_HTML_Entity_Reader.php
@@ -1,7 +1,5 @@
 <?php
 
-use WordPress\Data_Liberation\Block_Markup\WP_HTML_To_Blocks;
-
 /**
  * Converts a single HTML file into a stream of WordPress entities.
  *
@@ -9,14 +7,15 @@
  */
 class WP_HTML_Entity_Reader extends WP_Entity_Reader {
 
-	protected $html;
+	protected $html_processor;
 	protected $entities;
 	protected $finished = false;
 	protected $post_id;
+	protected $last_error;
 
-	public function __construct( $html, $post_id ) {
-		$this->html    = $html;
-		$this->post_id = $post_id;
+	public function __construct( $html_processor, $post_id ) {
+		$this->html_processor = $html_processor;
+		$this->post_id        = $post_id;
 	}
 
 	public function next_entity() {
@@ -36,8 +35,9 @@ public function next_entity() {
 		}
 
 		// We did not read any entities yet. Let's convert the HTML document into entities.
-		$converter = new WP_HTML_To_Blocks( $this->html );
+		$converter = new WP_HTML_To_Blocks( $this->html_processor );
 		if ( false === $converter->convert() ) {
+			$this->last_error = $converter->get_last_error();
 			return false;
 		}
 
@@ -90,6 +90,6 @@ public function is_finished(): bool {
 	}
 
 	public function get_last_error(): ?string {
-		return null;
+		return $this->last_error;
 	}
 }
diff --git a/packages/playground/data-liberation/src/xml-api/WP_XML_Processor.php b/packages/playground/data-liberation/src/xml-api/WP_XML_Processor.php
index 881e689020..22e3039818 100644
--- a/packages/playground/data-liberation/src/xml-api/WP_XML_Processor.php
+++ b/packages/playground/data-liberation/src/xml-api/WP_XML_Processor.php
@@ -1558,7 +1558,6 @@ private function parse_next_tag() {
 				 * See https://www.w3.org/TR/xml11.xml/#sec-cdata-sect
 				 */
 				if (
-					! $this->is_closing_tag &&
 					$doc_length > $this->token_starts_at + 8 &&
 					'[' === $xml[ $this->token_starts_at + 2 ] &&
 					'C' === $xml[ $this->token_starts_at + 3 ] &&
@@ -1583,6 +1582,59 @@ private function parse_next_tag() {
 					return true;
 				}
 
+				/*
+				 * Identify DOCTYPE nodes.
+				 *
+				 * See https://www.w3.org/TR/xml11.html/#dtd
+				 */
+				if (
+					$doc_length > $this->token_starts_at + 8 &&
+					'D' === $xml[ $at + 2 ] &&
+					'O' === $xml[ $at + 3 ] &&
+					'C' === $xml[ $at + 4 ] &&
+					'T' === $xml[ $at + 5 ] &&
+					'Y' === $xml[ $at + 6 ] &&
+					'P' === $xml[ $at + 7 ] &&
+					'E' === $xml[ $at + 8 ]
+				) {
+					$at += 9;
+					// Skip whitespace.
+					$at += strspn( $this->xml, " \t\f\r\n", $at );
+
+					if ( $doc_length <= $at ) {
+						$this->mark_incomplete_input( 'Unclosed DOCTYPE declaration.' );
+
+						return false;
+					}
+
+					// @TODO: Expose the "name" value instead of skipping it like that
+					$at += $this->parse_name( $at );
+
+					// Skip whitespace.
+					$at += strspn( $this->xml, " \t\f\r\n", $at );
+
+					if ( $doc_length <= $at ) {
+						$this->mark_incomplete_input( 'Unclosed DOCTYPE declaration.' );
+						return false;
+					}
+
+					if ( $this->xml[ $at ] !== '>' ) {
+						$this->last_error = self::ERROR_SYNTAX;
+						_doing_it_wrong(
+							__METHOD__,
+							__( 'Unsupported DOCTYPE syntax. Only a simple <!DOCTYPE name> is supported.' ),
+							'WP_VERSION'
+						);
+						return false;
+					}
+
+					$closer_at = $at;
+					$this->parser_state         = self::STATE_DOCTYPE_NODE;
+					$this->token_length         = $closer_at + 1 - $this->token_starts_at;
+					$this->bytes_already_parsed = $closer_at + 1;
+					return true;
+				}
+				
 				/*
 				 * Anything else here is either unsupported at this point or invalid
 				 * syntax. See the class-level @TODO annotations for more information.
@@ -1592,6 +1644,7 @@ private function parse_next_tag() {
 				return false;
 			}
 
+
 			/*
 			 * An `<?xml` token at the beginning of the document marks a start of an
 			 * xml declaration.
@@ -2471,6 +2524,22 @@ public function get_tag() {
 		return null;
 	}
 
+	/**
+	 * Indicates if the currently matched tag is expected to be closed.
+	 * Returns true for tag openers (<div>) and false for empty elements (<img />) and tag closers (</div>).
+	 *
+	 * This method exists to provide a consistent interface with WP_HTML_Processor.
+	 *
+	 * @return bool Whether the tag is expected to be closed.
+	 */
+	public function expects_closer() {
+		if ( self::STATE_MATCHED_TAG !== $this->parser_state ) {
+			return false;
+		}
+
+		return ! $this->is_empty_element() && ! $this->is_closing_tag;
+	}
+
 	/**
 	 * Indicates if the currently matched tag is an empty element tag.
 	 *
@@ -2604,6 +2673,9 @@ public function get_token_name() {
 			case self::STATE_CDATA_NODE:
 				return '#cdata-section';
 
+			case self::STATE_DOCTYPE_NODE:
+				return '#doctype';
+
 			case self::STATE_XML_DECLARATION:
 				return '#xml-declaration';
 
@@ -3030,10 +3102,11 @@ private function step_in_prolog( $node_to_process = self::PROCESS_NEXT_NODE ) {
 					$this->last_error = self::ERROR_SYNTAX;
 					_doing_it_wrong( __METHOD__, 'Unexpected token type in prolog stage.', 'WP_VERSION' );
 				}
-
 				return $this->step();
-			case '#xml-declaration':
+			// @TODO: Fail if there's more than one <!DOCTYPE> or if <!DOCTYPE> was found before the XML declaration token.
+			case '#doctype':
 			case '#comment':
+			case '#xml-declaration':
 			case '#processing-instructions':
 				return true;
 			case '#tag':
@@ -3393,6 +3466,18 @@ private function mark_incomplete_input(
 	 */
 	const STATE_CDATA_NODE = 'STATE_CDATA_NODE';
 
+	/**
+	 * Parser DOCTYPE Node State.
+	 *
+	 * Indicates that the parser has found a DOCTYPE declaration and it's possible
+	 * to read and modify its modifiable text.
+	 *
+	 * @since WP_VERSION
+	 *
+	 * @access private
+	 */
+	const STATE_DOCTYPE_NODE = 'STATE_DOCTYPE_NODE';
+
 	/**
 	 * Indicates that the parser has found an XML processing instruction.
 	 *
diff --git a/packages/playground/data-liberation/tests/WPEPubEntityReaderTests.php b/packages/playground/data-liberation/tests/WPEPubEntityReaderTests.php
index 4a6c7a2324..f6a4c205f4 100644
--- a/packages/playground/data-liberation/tests/WPEPubEntityReaderTests.php
+++ b/packages/playground/data-liberation/tests/WPEPubEntityReaderTests.php
@@ -21,6 +21,7 @@ public function test_entity_reader( $reader ) {
                 'data' => $data,
             ];
         }
+        $this->assertNull( $reader->get_last_error() );
         $this->assertEquals( 3, count($entities) );
         $this->assertEquals( 117, strlen($entities[0]['data']['content']) );
         $this->assertGreaterThan( 1000, strlen($entities[1]['data']['content']) );
diff --git a/packages/playground/data-liberation/tests/WPHTMLEntityReaderTests.php b/packages/playground/data-liberation/tests/WPHTMLEntityReaderTests.php
index be233599fa..f8d65c0357 100644
--- a/packages/playground/data-liberation/tests/WPHTMLEntityReaderTests.php
+++ b/packages/playground/data-liberation/tests/WPHTMLEntityReaderTests.php
@@ -13,7 +13,7 @@ public function test_entity_reader() {
 <h1>It is our pleasure to announce that WordPress 6.8 was released</h1>
 <p>Last week, WordPress 6.8 was released.</p>
 HTML;
-        $reader = new WP_HTML_Entity_Reader( $html, 1 );
+        $reader = new WP_HTML_Entity_Reader( new WP_HTML_Processor( $html ), 1 );
         $entities = [];
         while ( $reader->next_entity() ) {
             $data = $reader->get_entity()->get_data();
diff --git a/packages/playground/data-liberation/tests/WPHTMLToBlocksTests.php b/packages/playground/data-liberation/tests/WPHTMLToBlocksTests.php
index 66cae9670e..66a1a64306 100644
--- a/packages/playground/data-liberation/tests/WPHTMLToBlocksTests.php
+++ b/packages/playground/data-liberation/tests/WPHTMLToBlocksTests.php
@@ -16,7 +16,7 @@ public function test_metadata_extraction() {
 <h1>WordPress 6.8 was released</h1>
 <p>Last week, WordPress 6.8 was released. This release includes a new default theme, a new block editor experience, and a new block library. It also includes a new block editor experience, and a new block library.</p>
 HTML;
-        $converter = new WP_HTML_To_Blocks( $html );
+        $converter = new WP_HTML_To_Blocks( new WP_HTML_Processor( $html ) );
         $converter->convert( $html );
         $metadata = $converter->get_all_metadata();
         $expected_metadata = [
@@ -35,7 +35,7 @@ public function test_metadata_extraction() {
      * @dataProvider provider_test_conversion
      */
     public function test_html_to_blocks_conversion( $html, $expected ) {
-        $converter = new WP_HTML_To_Blocks( $html );
+        $converter = new WP_HTML_To_Blocks( new WP_HTML_Processor( $html ) );
         $converter->convert( $html );
         $blocks = $converter->get_block_markup();
 
@@ -136,7 +136,7 @@ public function provider_test_conversion() {
 
     public function test_html_to_blocks_excerpt() {
         $input = file_get_contents( __DIR__ . '/fixtures/html-to-blocks/excerpt.input.html' );
-        $converter = new WP_HTML_To_Blocks( $input );
+        $converter = new WP_HTML_To_Blocks( new WP_HTML_Processor( $input ) );
         $converter->convert( $input );
         $blocks = $converter->get_block_markup();
 
diff --git a/packages/playground/data-liberation/tests/WPXMLProcessorTests.php b/packages/playground/data-liberation/tests/WPXMLProcessorTests.php
index 2c3646dada..0e1dbf1ec4 100644
--- a/packages/playground/data-liberation/tests/WPXMLProcessorTests.php
+++ b/packages/playground/data-liberation/tests/WPXMLProcessorTests.php
@@ -1749,4 +1749,46 @@ public function test_pause_and_resume() {
 		$this->assertEquals( 'Hello there', $resumed->get_modifiable_text(), 'Did not find the expected text.' );
 	}
 
-}
\ No newline at end of file
+	/**
+	 * @ticket 61365
+	 * 
+	 * @covers WP_XML_Processor::next_token
+	 */
+	public function test_doctype_parsing() {
+		$processor = WP_XML_Processor::create_from_string(
+			'<!DOCTYPE html><root>Content</root>'
+		);
+
+		$this->assertTrue( $processor->next_token(), 'Did not find DOCTYPE node' );
+		$this->assertEquals( '#doctype', $processor->get_token_type(), 'Did not find DOCTYPE node' );
+		$this->assertTrue( $processor->next_token(), 'Did not find root tag' );
+		$this->assertEquals( 'root', $processor->get_tag(), 'Did not find root tag' );
+	}
+
+	/**
+	 * @ticket 61365
+	 * 
+	 * @covers WP_XML_Processor::next_token
+	 */
+	public function test_unsupported_doctype_parsing() {
+		$processor = WP_XML_Processor::create_from_string(
+			'<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"><root>Content</root>'
+		);
+
+		$this->assertFalse( $processor->next_token(), 'Did not reject complex DOCTYPE' );
+		$this->assertEquals( 'syntax', $processor->get_last_error(), 'Did not set syntax error' );
+	}
+
+	public function test_doctype_in_tag_content_is_syntax_error() {
+		$processor = WP_XML_Processor::create_from_string(
+			'<root>Content<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"></root>'
+		);
+
+		$processor->next_token();
+		$processor->next_token();
+
+		$this->assertFalse( $processor->next_token(), 'Did not reject DOCTYPE in tag content' );
+		$this->assertEquals( 'syntax', $processor->get_last_error(), 'Did not set syntax error' );
+	}
+
+}