Add basic conformance tests

Author: dmsnell

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

@@ -2615,20 +2615,27 @@ public function get_token_name() {
 	public function get_modifiable_text() {
 		$at     = $this->text_starts_at;
 		$length = $this->text_length;
+		$text   = substr( $this->html, $at, $length );
+		$text   = html_entity_decode( $text, ENT_QUOTES | ENT_HTML5 | ENT_SUBSTITUTE );
 
+		/*
+		 * PRE and TEXTAREA skip a leading newline, but this newline may not be a `\n`.
+		 * It could be a character reference, such as `<textarea>&#x0a;Content</textarea>`.
+		 * For these cases it's important to first decode the text content before checking
+		 * for a leading newline and removing it.
+		 */
 		if ( self::STATE_MATCHED_TAG === $this->parser_state ) {
 			switch ( $this->get_tag() ) {
 				case 'PRE':
 				case 'TEXTAREA':
-					if ( "\n" === $this->html[ $at ] ) {
-						++$at;
-						--$length;
+					if ( "\n" === $text[0] ) {
+						return substr( $text, 1 );
 					}
 					break;
 			}
 		}
 
-		return substr( $this->html, $at, $length );
+		return $text;
 	}
 
 	/**