From 14926c4cc991c4d9bc9e39e00d97f7d5830baf79 Mon Sep 17 00:00:00 2001
From: Jon Surrell <sirreal@users.noreply.github.com>
Date: Wed, 10 Jun 2026 11:06:38 +0200
Subject: [PATCH] HTML API: Limit leading newline serialization to HTML
 elements

---
 .../html-api/class-wp-html-processor.php      |  6 +-
 .../html-api/wpHtmlProcessor-serialize.php    | 82 +++++++++++++++++++
 2 files changed, 85 insertions(+), 3 deletions(-)

diff --git a/src/wp-includes/html-api/class-wp-html-processor.php b/src/wp-includes/html-api/class-wp-html-processor.php
index 35d91fad3129c..b0d3d53755da1 100644
--- a/src/wp-includes/html-api/class-wp-html-processor.php
+++ b/src/wp-includes/html-api/class-wp-html-processor.php
@@ -1461,8 +1461,8 @@ public function serialize_token(): string {
 
 		/*
 		 * The HTML parser strips a leading newline immediately after the start
-		 * tag of TEXTAREA, PRE, and LISTING elements. When serializing, prepend
-		 * a leading newline to ensure the semantic HTML content is preserved.
+		 * tag of HTML TEXTAREA, PRE, and LISTING elements. When serializing,
+		 * prepend a leading newline to ensure the semantic HTML content is preserved.
 		 *
 		 * For example, `<pre>\n\nX</pre>` must not become `<pre>\nX</pre>` because its content
 		 * has changed. However, `<pre>X</pre>` and `<pre>\nX</pre>` are _equivalent_.
@@ -1481,7 +1481,7 @@ public function serialize_token(): string {
 		 *
 		 * @see https://html.spec.whatwg.org/multipage/parsing.html
 		 */
-		if ( 'TEXTAREA' === $tag_name || 'PRE' === $tag_name || 'LISTING' === $tag_name ) {
+		if ( $in_html && ( 'TEXTAREA' === $tag_name || 'PRE' === $tag_name || 'LISTING' === $tag_name ) ) {
 			$html .= "\n";
 		}
 
diff --git a/tests/phpunit/tests/html-api/wpHtmlProcessor-serialize.php b/tests/phpunit/tests/html-api/wpHtmlProcessor-serialize.php
index e516addb6c314..fb11e414b8201 100644
--- a/tests/phpunit/tests/html-api/wpHtmlProcessor-serialize.php
+++ b/tests/phpunit/tests/html-api/wpHtmlProcessor-serialize.php
@@ -340,6 +340,88 @@ public function test_normalize_special_leading_newline_handling( string $input,
 		$this->assertEqualHTML( $expected, $normalized_twice );
 	}
 
+	/**
+	 * Ensures that the special leading newline rule applies only in the HTML namespace.
+	 *
+	 * @ticket 64607
+	 *
+	 * @dataProvider data_provider_special_leading_newline_namespace_serialization
+	 *
+	 * @param string $input    HTML input containing a PRE, LISTING, or TEXTAREA element.
+	 * @param string $expected Expected normalized output.
+	 */
+	public function test_special_leading_newline_rule_depends_on_namespace( string $input, string $expected ) {
+		$normalized = WP_HTML_Processor::normalize( $input );
+		$this->assertSame(
+			$expected,
+			$normalized,
+			'Should serialize special leading newlines according to the element namespace.'
+		);
+		$this->assertSame(
+			$expected,
+			WP_HTML_Processor::normalize( $normalized ),
+			'Normalizing already-normalized special leading newlines should not change them.'
+		);
+	}
+
+	/**
+	 * Data provider.
+	 *
+	 * @return array[]
+	 */
+	public static function data_provider_special_leading_newline_namespace_serialization() {
+		return array(
+			'MathML TEXTAREA'                            => array(
+				'<math><textarea>X</textarea></math>',
+				'<math><textarea>X</textarea></math>',
+			),
+			'MathML TEXTAREA with leading newline'       => array(
+				"<math><textarea>\nX</textarea></math>",
+				"<math><textarea>\nX</textarea></math>",
+			),
+			'SVG TEXTAREA'                               => array(
+				'<svg><textarea>X</textarea></svg>',
+				'<svg><textarea>X</textarea></svg>',
+			),
+			'SVG TEXTAREA with leading newline'          => array(
+				"<svg><textarea>\nX</textarea></svg>",
+				"<svg><textarea>\nX</textarea></svg>",
+			),
+			'HTML TEXTAREA inside SVG HTML integration point' => array(
+				'<svg><foreignObject><textarea>X</textarea></foreignObject></svg>',
+				"<svg><foreignObject><textarea>\nX</textarea></foreignObject></svg>",
+			),
+			'HTML TEXTAREA with leading newline inside SVG HTML integration point' => array(
+				"<svg><foreignObject><textarea>\n\nX</textarea></foreignObject></svg>",
+				"<svg><foreignObject><textarea>\n\nX</textarea></foreignObject></svg>",
+			),
+			'HTML TEXTAREA inside MathML text integration point' => array(
+				'<math><mtext><textarea>X</textarea></mtext></math>',
+				"<math><mtext><textarea>\nX</textarea></mtext></math>",
+			),
+			'HTML TEXTAREA with leading newline inside MathML text integration point' => array(
+				"<math><mtext><textarea>\n\nX</textarea></mtext></math>",
+				"<math><mtext><textarea>\n\nX</textarea></mtext></math>",
+			),
+			'HTML TEXTAREA inside MathML HTML integration point' => array(
+				'<math><annotation-xml encoding="text/html"><textarea>X</textarea></annotation-xml></math>',
+				"<math><annotation-xml encoding=\"text/html\"><textarea>\nX</textarea></annotation-xml></math>",
+			),
+			'HTML TEXTAREA with leading newline inside MathML HTML integration point' => array(
+				"<math><annotation-xml encoding=\"text/html\"><textarea>\n\nX</textarea></annotation-xml></math>",
+				"<math><annotation-xml encoding=\"text/html\"><textarea>\n\nX</textarea></annotation-xml></math>",
+			),
+			'HTML PRE after exiting SVG foreign content' => array(
+				'<svg><pre>X</pre></svg>',
+				"<svg></svg><pre>\nX</pre>",
+			),
+			'HTML LISTING after exiting MathML foreign content' => array(
+				'<math><listing>X</listing></math>',
+				"<math></math><listing>\nX</listing>",
+			),
+		);
+	}
+
 	/**
 	 * Ensures that fuzzer-discovered inputs do not emit native PHP errors.
 	 *