Add test, fix list item scope logic

src/wp-includes/html-api/class-wp-html-open-elements.php

@@ -129,7 +129,7 @@ public function has_element_in_specific_scope( $tag_name, $termination_list ) {
 			}
 
 			if ( in_array( $node->node_name, $termination_list, true ) ) {
-				return true;
+				return false;
 			}
 		}
 

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

@@ -821,10 +821,21 @@ private function step_in_body() {
 			case '-DT':
 			case '-LI':
 				if (
+					/*
+					 * An end tag whose tag name is "li":
+					 * If the stack of open elements does not have an li element in list item scope,
+					 * then this is a parse error; ignore the token.
+					 */
 					(
 						'LI' === $tag_name &&
 						! $this->state->stack_of_open_elements->has_element_in_list_item_scope( 'LI' )
 					) ||
+					/*
+					 * An end tag whose tag name is one of: "dd", "dt":
+					 * If the stack of open elements does not have an element in scope that is an
+					 * HTML element with the same tag name as that of the token, then this is a
+					 * parse error; ignore the token.
+					 */
 					(
 						'LI' !== $tag_name &&
 						! $this->state->stack_of_open_elements->has_element_in_scope( $tag_name )

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

@@ -323,4 +323,26 @@ public function test_in_body_dt_in_dt_closes_p_in_button_scope() {
 			'DT should have left the outer P open, but closed it.'
 		);
 	}
+
+	public function test_unexpected_li_close_tag_is_properly_contained() {
+		$processor = WP_HTML_Processor::create_fragment( '<ul><li><ul></li><li target>a</li></ul></li></ul>' );
+
+		while (
+			null === $processor->get_attribute( 'target' ) &&
+			$processor->next_tag()
+		) {
+			continue;
+		}
+
+		$this->assertTrue(
+			$processor->get_attribute( 'target' ),
+			'Failed to find target node.'
+		);
+
+		$this->assertSame(
+			array( 'HTML', 'BODY', 'UL', 'LI', 'UL', 'LI' ),
+			$processor->get_breadcrumbs(),
+			'Unexpected LI close tag should have left its containing UL open, but closed it.'
+		);
+	}
 }