Plugin Tweaks: Add a gutenberg tweaks file to fix "level 0" headings

mu-plugins/plugin-tweaks/gutenberg.php

@@ -0,0 +1,32 @@
+<?php
+
+namespace WordPressdotorg\MU_Plugins\Plugin_Tweaks\Gutenberg;
+
+defined( 'WPINC' ) || die();
+
+/**
+ * Actions and filters.
+ */
+add_filter( 'render_block_core/post-title', __NAMESPACE__ . '\swap_h0_for_paragraph', 20 );
+add_filter( 'render_block_core/query-title', __NAMESPACE__ . '\swap_h0_for_paragraph', 20 );
+
+/**
+ * Replace invalid `h0` tags with paragraphs.
+ *
+ * Setting the `level` to 0 technically works for site-title, post-title,
+ * and query-title, but the latter two don't do any validation before outputting
+ * `<h{level}>`, so we end up with the invalid `h0` when trying to remove
+ * heading semantics.
+ *
+ * @param string $block_content The block content.
+ *
+ * @return string The updated block content.
+ */
+function swap_h0_for_paragraph( $block_content ) {
+	return str_replace(
+		array( '<h0', '</h0>' ),
+		array( '<p', '</p>' ),
+		$block_content
+	);
+}
+

mu-plugins/plugin-tweaks/index.php

@@ -12,3 +12,4 @@
 require_once __DIR__ . '/wporg-internal-notes.php';
 require_once __DIR__ . '/stream.php';
 require_once __DIR__ . '/incompatible-plugins.php';
+require_once __DIR__ . '/gutenberg.php';