diff --git a/src/wp-includes/html-api/class-wp-html-tag-processor.php b/src/wp-includes/html-api/class-wp-html-tag-processor.php
index 7e047c4808081..4a6abf65846d6 100644
--- a/src/wp-includes/html-api/class-wp-html-tag-processor.php
+++ b/src/wp-includes/html-api/class-wp-html-tag-processor.php
@@ -1232,12 +1232,12 @@ public function class_list() {
 			$at += $length;
 
 			/*
-			 * It's expected that the number of class names for a given tag is relatively small.
-			 * Given this, it is probably faster overall to scan an array for a value rather
-			 * than to use the class name as a key and check if it's a key of $seen.
+			 * `$seen` is keyed by class name. PHP stores a canonical integer string
+			 * such as "123" as the integer key 123, and no other string maps to that
+			 * key, so distinct names stay distinct.
 			 */
-			if ( ! in_array( $name, $seen, true ) ) {
-				$seen[] = $name;
+			if ( ! isset( $seen[ $name ] ) ) {
+				$seen[ $name ] = true;
 				yield $name;
 			}
 
@@ -2502,19 +2502,24 @@ private function class_name_updates_to_attributes_updates(): void {
 		 */
 		$modified = false;
 
+		/*
+		 * `$seen` and `$to_remove` are keyed by class name. PHP stores a canonical
+		 * integer string such as "123" as the integer key 123, and no other string
+		 * maps to that key, so distinct names stay distinct.
+		 */
 		$seen      = array();
 		$to_remove = array();
 		$is_quirks = self::QUIRKS_MODE === $this->compat_mode;
 		if ( $is_quirks ) {
 			foreach ( $this->classname_updates as $updated_name => $action ) {
 				if ( self::REMOVE_CLASS === $action ) {
-					$to_remove[] = strtolower( $updated_name );
+					$to_remove[ strtolower( $updated_name ) ] = true;
 				}
 			}
 		} else {
 			foreach ( $this->classname_updates as $updated_name => $action ) {
 				if ( self::REMOVE_CLASS === $action ) {
-					$to_remove[] = $updated_name;
+					$to_remove[ $updated_name ] = true;
 				}
 			}
 		}
@@ -2539,17 +2544,17 @@ private function class_name_updates_to_attributes_updates(): void {
 			$at                   += $name_length;
 
 			// If this class is marked for removal, remove it and move on to the next one.
-			if ( in_array( $comparable_class_name, $to_remove, true ) ) {
+			if ( isset( $to_remove[ $comparable_class_name ] ) ) {
 				$modified = true;
 				continue;
 			}
 
 			// If a class has already been seen then skip it; it should not be added twice.
-			if ( in_array( $comparable_class_name, $seen, true ) ) {
+			if ( isset( $seen[ $comparable_class_name ] ) ) {
 				continue;
 			}
 
-			$seen[] = $comparable_class_name;
+			$seen[ $comparable_class_name ] = true;
 
 			/*
 			 * Otherwise, append it to the new "class" attribute value.
@@ -2572,7 +2577,7 @@ private function class_name_updates_to_attributes_updates(): void {
 		// Add new classes by appending those which haven't already been seen.
 		foreach ( $this->classname_updates as $name => $operation ) {
 			$comparable_name = $is_quirks ? strtolower( $name ) : $name;
-			if ( self::ADD_CLASS === $operation && ! in_array( $comparable_name, $seen, true ) ) {
+			if ( self::ADD_CLASS === $operation && ! isset( $seen[ $comparable_name ] ) ) {
 				$modified = true;
 
 				$class .= strlen( $class ) > 0 ? ' ' : '';
diff --git a/tests/phpunit/tests/html-api/wpHtmlTagProcessor.php b/tests/phpunit/tests/html-api/wpHtmlTagProcessor.php
index 1bfaa01bf0453..0647fb634d1d5 100644
--- a/tests/phpunit/tests/html-api/wpHtmlTagProcessor.php
+++ b/tests/phpunit/tests/html-api/wpHtmlTagProcessor.php
@@ -2691,6 +2691,7 @@ public function test_class_list_null_bytes_replaced() {
 	/**
 	 * @ticket 59209
 	 * @ticket 65846
+	 * @ticket 65967
 	 *
 	 * @covers WP_HTML_Tag_Processor::class_list
 	 *
@@ -2742,6 +2743,8 @@ public static function data_class_list(): array {
 			'Decimal reference without semicolon'     => array( "<div class='a&#32b'>", array( 'a', 'b' ) ),
 			'Hex reference without semicolon'         => array( "<div class='a&#x20g'>", array( 'a', 'g' ) ),
 			'Bare ampersand'                          => array( "<div class='&'>", array( '&' ) ),
+			'Numeric-looking names stay distinct'     => array( "<div class='123 0123 1e3 -0 0'>", array( '123', '0123', '1e3', '-0', '0' ) ),
+			'Numeric duplicates collapse'             => array( "<div class='123 123'>", array( '123' ) ),
 		);
 	}
 
@@ -2800,6 +2803,33 @@ public static function data_html_with_variations_of_class_values_and_sought_clas
 		);
 	}
 
+	/**
+	 * Ensures that add_class() and remove_class() match class names which are
+	 * canonical integer strings, such as "123", and only those.
+	 *
+	 * @ticket 65967
+	 *
+	 * @covers WP_HTML_Tag_Processor::add_class
+	 * @covers WP_HTML_Tag_Processor::remove_class
+	 */
+	public function test_add_class_and_remove_class_match_numeric_class_names() {
+		$processor = new WP_HTML_Tag_Processor( '<div class="123 abc 123">' );
+		$processor->next_tag();
+		$processor->remove_class( '123' );
+
+		$this->assertSame( '<div class="abc">', $processor->get_updated_html(), 'Failed to remove class name "123".' );
+
+		$processor = new WP_HTML_Tag_Processor( '<div class="123 0123 1e3">' );
+		$processor->next_tag();
+		$processor->add_class( '123' );
+
+		$this->assertSame( '<div class="123 0123 1e3">', $processor->get_updated_html(), 'Added a second copy of class name "123".' );
+
+		$processor->remove_class( '0123' );
+
+		$this->assertSame( '<div class="123 1e3">', $processor->get_updated_html(), 'Did not remove class name "0123" alone.' );
+	}
+
 	/**
 	 * Ensures that the invalid comment closing syntax "--!>" properly closes a comment.
 	 *