Add explanatory comment

Author: sirreal

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

@@ -1412,6 +1412,28 @@ public function serialize_token(): string {
 
 		$html .= '>';
 
+		/*
+		 * The HTML parser strips a leading newline immediately after the start
+		 * tag of TEXTAREA, PRE, and LISTING elements. When serializing, prepend
+		 * a leading newline to ensure the semantic HTML content is preserved.
+		 *
+		 * For example, `<pre>\n\nX</pre>` must not become `<pre>\nX</pre>` because its content
+		 * has changed. However, `<pre>X</pre>` and `<pre>\nX</pre>` are _equivalent_.
+		 *
+		 * > A start tag whose tag name is "textarea"
+		 * >   …
+		 * >   If the next token is a U+000A LINE FEED (LF) character token, then ignore
+		 * >   that token and move on to the next one. (Newlines at the start of textarea
+		 * >   elements are ignored as an authoring convenience.)
+		 *
+		 * > A start tag whose tag name is one of: "pre", "listing"
+		 * >   …
+		 * >   If the next token is a U+000A LINE FEED (LF) character token, then ignore
+		 * >   that token and move on to the next one. (Newlines at the start of pre blocks
+		 * >   are ignored as an authoring convenience.)
+		 *
+		 * @see https://html.spec.whatwg.org/multipage/parsing.html
+		 */
 		if ( $tag_name === 'TEXTAREA' || $tag_name === 'PRE' || $tag_name === 'LISTING' ) {
 			$html .= "\n";
 		}