diff --git a/src/wp-includes/formatting.php b/src/wp-includes/formatting.php
index ecae45083a6ca..95c5acdf6e35d 100644
--- a/src/wp-includes/formatting.php
+++ b/src/wp-includes/formatting.php
@@ -660,13 +660,33 @@ function get_html_split_regex() {
 			.     $cdata
 			. ')';
 
+		// Keep in sync with the element pattern in _get_wptexturize_split_regex().
+		$element =
+			'(?(?=[a-zA-Z])'                   // Does a tag name follow?
+			.     '(?:'                        // Unroll the loop: Consume everything until > is found.
+			.         '[^>=]++'                // Consume anything but > and =.
+			.     '|'
+			.         '=[ \t\f\r\n]*'          // Attribute value follows.
+			.         '(?:'
+			.             '"[^"]*"'            // Double-quoted value, which may contain >.
+			.         '|'
+			.             '\'[^\']*\''         // Single-quoted value, which may contain >.
+			.         '|'
+			.             '[^> \t\f\r\n]*+'    // Unquoted value.
+			.         ')'
+			.     ')*+'                        // Loop possessively.
+			. '|'                              // ...else not a tag, so quotes are not special.
+			.     '[^>]*+'
+			. ')'
+			. '>?';                            // End of element. If not found, match all input.
+
 		$regex =
 			'/('                // Capture the entire match.
 			.     '<'           // Find start of element.
 			.     '(?'          // Conditional expression follows.
 			.         $escaped  // Find end of escaped element.
 			.     '|'           // ...else...
-			.         '[^>]*>?' // Find end of normal element.
+			.         $element  // Find end of element, allowing > inside quoted attribute values.
 			.     ')'
 			. ')/';
 		// phpcs:enable
@@ -699,12 +719,32 @@ function _get_wptexturize_split_regex( $shortcode_regex = '' ) {
 			. ')*+'         // Loop possessively.
 			. '(?:-->)?';   // End of comment. If not found, match all input.
 
+		// Keep in sync with the element pattern in get_html_split_regex().
+		$element =
+			'(?(?=[a-zA-Z])'                   // Does a tag name follow?
+			.     '(?:'                        // Unroll the loop: Consume everything until > is found.
+			.         '[^>=]++'                // Consume anything but > and =.
+			.     '|'
+			.         '=[ \t\f\r\n]*'          // Attribute value follows.
+			.         '(?:'
+			.             '"[^"]*"'            // Double-quoted value, which may contain >.
+			.         '|'
+			.             '\'[^\']*\''         // Single-quoted value, which may contain >.
+			.         '|'
+			.             '[^> \t\f\r\n]*+'    // Unquoted value.
+			.         ')'
+			.     ')*+'                        // Loop possessively.
+			. '|'                              // ...else not a tag, so quotes are not special.
+			.     '[^>]*+'
+			. ')'
+			. '>?';                            // End of element. If not found, match all input.
+
 		$html_regex = // Needs replaced with wp_html_split() per Shortcode API Roadmap.
 			'<'                  // Find start of element.
 			. '(?(?=!--)'        // Is this a comment?
 			.     $comment_regex // Find end of comment.
 			. '|'
-			.     '[^>]*>?'      // Find end of element. If not found, match all input.
+			.     $element       // Find end of element.
 			. ')';
 		// phpcs:enable
 	}
diff --git a/tests/phpunit/tests/formatting/wpHtmlSplit.php b/tests/phpunit/tests/formatting/wpHtmlSplit.php
index 5635b3a9a3b27..8adb03adb163c 100644
--- a/tests/phpunit/tests/formatting/wpHtmlSplit.php
+++ b/tests/phpunit/tests/formatting/wpHtmlSplit.php
@@ -37,6 +37,138 @@ public static function data_basic_features() {
 		);
 	}
 
+	/**
+	 * Ensure that > inside quoted attribute values does not cause premature
+	 * tag splitting in wp_html_split().
+	 *
+	 * @ticket 63997
+	 * @ticket 57381
+	 * @ticket 63426
+	 *
+	 * @dataProvider data_gt_in_quoted_attribute_values
+	 *
+	 * @param non-falsy-string $input  Input HTML.
+	 * @param list<string>     $output Expected split.
+	 */
+	public function test_gt_in_quoted_attribute_values( string $input, array $output ): void {
+		$this->assertSame( $output, wp_html_split( $input ) );
+	}
+
+	/**
+	 * Data provider for {@see self::test_gt_in_quoted_attribute_values()}.
+	 *
+	 * @return list<array{ 0: non-falsy-string, 1: list<string> }>
+	 */
+	public static function data_gt_in_quoted_attribute_values(): array {
+		return array(
+			array(
+				'<div data-test="a > b">content</div>',
+				array( '', '<div data-test="a > b">', 'content', '</div>', '' ),
+			),
+			array(
+				'<div data-test=\'a > b\'>content</div>',
+				array( '', '<div data-test=\'a > b\'>', 'content', '</div>', '' ),
+			),
+			array(
+				'<div data-test="a > b" data-other="c &gt; d">content</div>',
+				array( '', '<div data-test="a > b" data-other="c &gt; d">', 'content', '</div>', '' ),
+			),
+			array(
+				'<div data-template="Label <% value %>" data-label="{A \'message\' string}">Content</div>',
+				array( '', '<div data-template="Label <% value %>" data-label="{A \'message\' string}">', 'Content', '</div>', '' ),
+			),
+			array(
+				'<iframe srcdoc="<body></body>"></iframe>',
+				array( '', '<iframe srcdoc="<body></body>">', '', '</iframe>', '' ),
+			),
+			array(
+				'<div class="swiper [&>.swiper-pagination]:static">content</div>',
+				array( '', '<div class="swiper [&>.swiper-pagination]:static">', 'content', '</div>', '' ),
+			),
+		);
+	}
+
+	/**
+	 * Ensure that a quote in an unquoted attribute value does not pair with a
+	 * quote after the tag.
+	 *
+	 * @ticket 63997
+	 */
+	public function test_quote_in_unquoted_attribute_value_does_not_pair_across_tag(): void {
+		$input = "<p>Hi</p>\n<a href=\"/x\" title=it's>It's here</a>";
+
+		$this->assertSame(
+			array( '', '<p>', 'Hi', '</p>', "\n", '<a href="/x" title=it\'s>', 'It\'s here', '</a>', '' ),
+			wp_html_split( $input )
+		);
+	}
+
+	/**
+	 * Ensure that an unclosed tag matches the remaining input.
+	 *
+	 * @ticket 63997
+	 */
+	public function test_unclosed_tag_matches_remaining_input(): void {
+		$input = "<p>Truncated <a href=\"http://example.com/it's";
+
+		$this->assertSame(
+			array( '', '<p>', 'Truncated ', '<a href="http://example.com/it\'s', '' ),
+			wp_html_split( $input )
+		);
+	}
+
+	/**
+	 * Ensure that a quote in an attribute name does not extend the tag past the
+	 * next >.
+	 *
+	 * @ticket 63997
+	 */
+	public function test_quote_in_attribute_name_does_not_extend_tag(): void {
+		$this->assertSame(
+			array( '', '<div "=noquote >', ' id="quote">' ),
+			wp_html_split( '<div "=noquote > id="quote">' )
+		);
+	}
+
+	/**
+	 * Ensure that a quote-named attribute splits the same as before.
+	 *
+	 * @ticket 63997
+	 */
+	public function test_quote_named_attribute_is_one_element(): void {
+		$this->assertSame(
+			array( '', '<div "=noquote id="quote">', 'x' ),
+			wp_html_split( '<div "=noquote id="quote">x' )
+		);
+	}
+
+	/**
+	 * Ensure that a quote after a < that does not start a tag does not pair
+	 * with a quote after the next >.
+	 *
+	 * @ticket 63997
+	 */
+	public function test_quote_after_non_tag_lt_does_not_pair_across_gt(): void {
+		$this->assertSame(
+			array( '', '<p>', 'x', "<=' then</p>", '', '<p>', "don't", '</p>', '' ),
+			wp_html_split( "<p>x<=' then</p><p>don't</p>" )
+		);
+	}
+
+	/**
+	 * Ensure that a CDATA section containing > is a single element.
+	 *
+	 * @ticket 63997
+	 */
+	public function test_cdata_with_gt_is_one_element(): void {
+		$input = '<script>/*<![CDATA[*/ if (a > b && c) {} /*]]>*/</script>';
+
+		$this->assertSame(
+			array( '', '<script>', '/*', '<![CDATA[*/ if (a > b && c) {} /*]]>', '*/', '</script>', '' ),
+			wp_html_split( $input )
+		);
+	}
+
 	/**
 	 * Automated performance testing of the main regex.
 	 *
diff --git a/tests/phpunit/tests/formatting/wpTexturize.php b/tests/phpunit/tests/formatting/wpTexturize.php
index 61b8b46c46a26..91a7bc4e7ed68 100644
--- a/tests/phpunit/tests/formatting/wpTexturize.php
+++ b/tests/phpunit/tests/formatting/wpTexturize.php
@@ -2111,6 +2111,123 @@ public function test_trailing_less_than() {
 		$this->assertSame( 'F&#8211;oo<', wptexturize( 'F--oo<', true ) );
 	}
 
+	/**
+	 * Ensure that > inside quoted attribute values does not cause premature
+	 * tag splitting, which would leave quotes exposed to texturization.
+	 *
+	 * @ticket 63997
+	 * @ticket 57381
+	 * @ticket 63426
+	 *
+	 * @dataProvider data_gt_in_quoted_attribute_values
+	 *
+	 * @param string $input  Input HTML.
+	 * @param string $output Expected output.
+	 */
+	public function test_gt_in_quoted_attribute_values( string $input, string $output ): void {
+		$this->assertSame( $output, wptexturize( $input ) );
+	}
+
+	/**
+	 * Data provider for {@see self::test_gt_in_quoted_attribute_values()}.
+	 *
+	 * @return list<array{0: string, 1: string}>
+	 */
+	public static function data_gt_in_quoted_attribute_values(): array {
+		return array(
+			array(
+				'<div data-test=\'a > 0 ? "yes" : "no"\'>content</div>',
+				'<div data-test=\'a > 0 ? "yes" : "no"\'>content</div>',
+			),
+			array(
+				"<div data-test=\"a > 0 ? 'yes' : 'no'\">content</div>",
+				"<div data-test=\"a > 0 ? 'yes' : 'no'\">content</div>",
+			),
+			array(
+				'<div data-a="x > y" data-b="z">content</div>',
+				'<div data-a="x > y" data-b="z">content</div>',
+			),
+			array(
+				'<div data-test=\'a &gt; 0 ? "yes" : "no"\'>content</div>',
+				'<div data-test=\'a &gt; 0 ? "yes" : "no"\'>content</div>',
+			),
+			array(
+				'<div data-template="Label <% value %>" data-label="{A \'message\' string}">Content</div>',
+				'<div data-template="Label <% value %>" data-label="{A \'message\' string}">Content</div>',
+			),
+			array(
+				'<iframe srcdoc="<body></body>"></iframe>',
+				'<iframe srcdoc="<body></body>"></iframe>',
+			),
+			array(
+				'<div class="swiper [&>.swiper-pagination]:static">content</div>',
+				'<div class="swiper [&#038;>.swiper-pagination]:static">content</div>',
+			),
+		);
+	}
+
+	/**
+	 * Ensure that an ampersand in a quoted attribute value is still encoded.
+	 *
+	 * @ticket 57381
+	 */
+	public function test_ampersand_in_quoted_attribute_value_is_encoded(): void {
+		$this->assertSame(
+			'<a href="?a[]=1&#038;a[]=2">x</a>',
+			wptexturize( '<a href="?a[]=1&a[]=2">x</a>' )
+		);
+	}
+
+	/**
+	 * Ensure that a quote in an unquoted attribute value does not pair with a
+	 * quote after the tag and swallow the following text.
+	 *
+	 * @ticket 63997
+	 */
+	public function test_quote_in_unquoted_attribute_value_does_not_pair_across_tag(): void {
+		$input = "<p>Hi</p>\n<a href=\"/x\" title=it's>It's here</a>";
+
+		$this->assertSame(
+			"<p>Hi</p>\n<a href=\"/x\" title=it's>It&#8217;s here</a>",
+			wptexturize( $input )
+		);
+	}
+
+	/**
+	 * Ensure that a quote after a < that does not start a tag does not pair
+	 * with a quote in the following text.
+	 *
+	 * @ticket 63997
+	 */
+	public function test_quote_after_non_tag_lt_does_not_pair_across_gt(): void {
+		$this->assertSame(
+			"<p>x<=' then</p><p>don&#8217;t</p>",
+			wptexturize( "<p>x<=' then</p><p>don't</p>" )
+		);
+	}
+
+	/**
+	 * Ensure that an unclosed tag is not texturized.
+	 *
+	 * @ticket 63997
+	 */
+	public function test_unclosed_tag_is_not_texturized(): void {
+		$input = "<p>Truncated <a href=\"http://example.com/it's";
+
+		$this->assertSame( $input, wptexturize( $input ) );
+	}
+
+	/**
+	 * Ensure that an ampersand inside a CDATA section is not encoded.
+	 *
+	 * @ticket 63997
+	 */
+	public function test_ampersand_in_cdata_is_not_encoded(): void {
+		$input = '<script>/*<![CDATA[*/ if (a > b && c) {} /*]]>*/</script>';
+
+		$this->assertSame( $input, wptexturize( $input ) );
+	}
+
 	public static function data_whole_posts() {
 		require_once DIR_TESTDATA . '/formatting/whole-posts.php';
 		return data_whole_posts();