Add PLAINTEXT support.

Author: dmsnell

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

@@ -2122,13 +2122,8 @@ private function step_in_body(): bool {
 					$this->close_a_p_element();
 				}
 
-				/*
-				 * @todo This may need to be handled in the Tag Processor and turn into
-				 *       a single self-contained tag like TEXTAREA, whose modifiable text
-				 *       is the rest of the input document as plaintext.
-				 */
-				$this->bail( 'Cannot process PLAINTEXT elements.' );
-				break;
+				$this->insert_html_element( $this->state->current_token );
+				return true;
 
 			/*
 			 * > A start tag whose tag name is "button"

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

@@ -974,6 +974,15 @@ private function base_class_next_token(): bool {
 			return true;
 		}
 
+		if ( 'PLAINTEXT' === $tag_name ) {
+			$doc_length                 = strlen( $this->html );
+			$this->text_starts_at       = $this->token_starts_at + $this->token_length;
+			$this->text_length          = $doc_length - $this->text_starts_at;
+			$this->token_length         = $doc_length - $this->token_starts_at;
+			$this->bytes_already_parsed = $doc_length;
+			return true;
+		}
+
 		/*
 		 * There are certain elements whose children are not DATA but are instead
 		 * RCDATA or RAWTEXT. These cannot contain other elements, and the contents
@@ -2919,6 +2928,9 @@ public function get_modifiable_text(): string {
 
 		$tag_name = $this->get_token_name();
 		if (
+			// Plaintext data is not decoded.
+			'PLAINTEXT' === $tag_name ||
+
 			// Script data is not decoded.
 			'SCRIPT' === $tag_name ||
 
@@ -3533,7 +3545,13 @@ private function matches(): bool {
 		}
 
 		// Does the tag name match the requested tag name in a case-insensitive manner?
-		if ( isset( $this->sought_tag_name ) && 0 !== substr_compare( $this->html, $this->sought_tag_name, $this->tag_name_starts_at, $this->tag_name_length, true ) ) {
+		if (
+			isset( $this->sought_tag_name ) &&
+			(
+				strlen( $this->sought_tag_name ) !== $this->tag_name_length ||
+				0 !== substr_compare( $this->html, $this->sought_tag_name, $this->tag_name_starts_at, $this->tag_name_length, true )
+			)
+		) {
 			return false;
 		}