Implement HR tag handling

src/wp-includes/html-api/class-wp-html-processor.php

@@ -941,6 +941,17 @@ private function step_in_body() {
 				$this->reconstruct_active_formatting_elements();
 				$this->insert_html_element( $this->state->current_token );
 				return true;
+
+			/*
+			 * > A start tag whose tag name is "hr"
+			 */
+			case '+HR':
+				if ( $this->state->stack_of_open_elements->has_p_in_button_scope() ) {
+					$this->close_a_p_element();
+				}
+				$this->insert_html_element( $this->state->current_token );
+				$this->state->frameset_ok = false;
+				return true;
 		}
 
 		/*
@@ -977,7 +988,6 @@ private function step_in_body() {
 			case 'FRAME':
 			case 'FRAMESET':
 			case 'HEAD':
-			case 'HR':
 			case 'HTML':
 			case 'IFRAME':
 			case 'INPUT':

tests/phpunit/tests/html-api/wpHtmlProcessor.php

@@ -173,7 +173,6 @@ public function data_unsupported_special_in_body_tags() {
 			'FRAME'     => array( 'FRAME' ),
 			'FRAMESET'  => array( 'FRAMESET' ),
 			'HEAD'      => array( 'HEAD' ),
-			'HR'        => array( 'HR' ),
 			'HTML'      => array( 'HTML' ),
 			'IFRAME'    => array( 'IFRAME' ),
 			'INPUT'     => array( 'INPUT' ),

tests/phpunit/tests/html-api/wpHtmlProcessorBreadcrumbs.php

@@ -175,7 +175,6 @@ public function data_unsupported_elements() {
 			'FRAME',
 			'FRAMESET',
 			'HEAD',
-			'HR',
 			'HTML',
 			'IFRAME',
 			'INPUT',