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..244b95dc9c7c0 100644
--- a/src/wp-includes/html-api/class-wp-html-processor.php
+++ b/src/wp-includes/html-api/class-wp-html-processor.php
@@ -5474,6 +5474,17 @@ public function get_attribute_names_with_prefix( $prefix ): ?array {
 	/**
 	 * Adds a new class name to the currently matched tag.
 	 *
+	 * Whitespace in `$class_name` is preserved verbatim. This may result
+	 * in multiple class names being added to the element's class list.
+	 *
+	 * Examples:
+	 *
+	 *     $p->add_class( 'wp-block' );
+	 *     // Adds one class: "wp-block".
+	 *
+	 *     $p->add_class( 'wp-block alignwide' );
+	 *     // Adds two classes: "wp-block" and "alignwide".
+	 *
 	 * @since 6.6.0 Subclassed for the HTML Processor.
 	 *
 	 * @param string $class_name The class name to add.
@@ -5487,6 +5498,7 @@ public function add_class( $class_name ): bool {
 	 * Removes a class name from the currently matched tag.
 	 *
 	 * @since 6.6.0 Subclassed for the HTML Processor.
+	 * @since 7.1.0 Returns false when `$class_name` contains ASCII whitespace.
 	 *
 	 * @param string $class_name The class name to remove.
 	 * @return bool Whether the class was set to be removed.
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 77c1a471db5b1..53a207ec04cda 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
@@ -1242,6 +1242,15 @@ public function has_class( $wanted_class ): ?bool {
 			return null;
 		}
 
+		if ( false !== strpbrk( $wanted_class, " \t\f\r\n" ) ) {
+			_doing_it_wrong(
+				__METHOD__,
+				__( 'A class name cannot contain ASCII whitespace.' ),
+				'7.1.0'
+			);
+			return false;
+		}
+
 		$case_insensitive = self::QUIRKS_MODE === $this->compat_mode;
 
 		$wanted_length = strlen( $wanted_class );
@@ -4537,6 +4546,17 @@ public function remove_attribute( $name ): bool {
 	/**
 	 * Adds a new class name to the currently matched tag.
 	 *
+	 * Whitespace in `$class_name` is preserved verbatim. This may result
+	 * in multiple class names being added to the element's class list.
+	 *
+	 * Examples:
+	 *
+	 *     $p->add_class( 'wp-block' );
+	 *     // Adds one class: "wp-block".
+	 *
+	 *     $p->add_class( 'wp-block alignwide' );
+	 *     // Adds two classes: "wp-block" and "alignwide".
+	 *
 	 * @since 6.2.0
 	 *
 	 * @param string $class_name The class name to add.
@@ -4580,6 +4600,7 @@ public function add_class( $class_name ): bool {
 	 * Removes a class name from the currently matched tag.
 	 *
 	 * @since 6.2.0
+	 * @since 7.1.0 Returns false when `$class_name` contains ASCII whitespace.
 	 *
 	 * @param string $class_name The class name to remove.
 	 * @return bool Whether the class was set to be removed.
@@ -4592,6 +4613,15 @@ public function remove_class( $class_name ): bool {
 			return false;
 		}
 
+		if ( false !== strpbrk( $class_name, " \t\f\r\n" ) ) {
+			_doing_it_wrong(
+				__METHOD__,
+				__( 'A class name cannot contain ASCII whitespace.' ),
+				'7.1.0'
+			);
+			return false;
+		}
+
 		if ( self::QUIRKS_MODE !== $this->compat_mode ) {
 			$this->classname_updates[ $class_name ] = self::REMOVE_CLASS;
 			return true;