diff --git a/src/wp-includes/interactivity-api/class-wp-interactivity-api.php b/src/wp-includes/interactivity-api/class-wp-interactivity-api.php
index 2190e125d08aa..506abd01537ab 100644
--- a/src/wp-includes/interactivity-api/class-wp-interactivity-api.php
+++ b/src/wp-includes/interactivity-api/class-wp-interactivity-api.php
@@ -183,8 +183,9 @@ final class WP_Interactivity_API {
 
 	/**
 	 * Marker that makes the placeholders of the current `process_directives()`
-	 * call recognizable. It occurs nowhere in the HTML that call received, and
-	 * it differs from the marker of any call that the current one runs inside.
+	 * call recognizable, or an empty string until the first `data-wp-html` host
+	 * of the call renders and draws it. It holds a random UUID, so no text
+	 * written before the call can realistically hold it.
 	 *
 	 * @since 7.2.0
 	 * @var string
@@ -536,17 +537,11 @@ public function process_directives( string $html ): string {
 		/*
 		 * The trusted HTML of `data-wp-html` hosts never takes part in the
 		 * processing: each host receives a placeholder comment, and the comments
-		 * are replaced by their HTML once the result is known. The marker that
-		 * identifies them is chosen so that it occurs nowhere in the input, which
-		 * makes it impossible for the input to contain a placeholder. It is also
-		 * longer than the marker of any call this one runs inside, so their
-		 * placeholders never coincide.
+		 * are replaced by their HTML once the result is known. The call starts
+		 * without the marker that identifies them: the first host that renders
+		 * draws it, so a call without such a host does no marker work.
 		 */
-		$marker = '' === $this->html_placeholder_marker ? 'wp-interactivity-dangerous-html' : $this->html_placeholder_marker . '_';
-		while ( str_contains( $html, $marker ) ) {
-			$marker .= '_';
-		}
-		$this->html_placeholder_marker = $marker;
+		$this->html_placeholder_marker = '';
 		$this->html_placeholders       = array();
 
 		try {
@@ -1698,8 +1693,13 @@ private function data_wp_html_processor( WP_Interactivity_API_Directives_Process
 		/*
 		 * The HTML is held back and a placeholder comment takes its place, so no
 		 * later step of the call scans it. A comment is a single token for the
-		 * processors, and the marker in it occurs nowhere in the input.
+		 * processors. Its marker holds a random UUID, drawn once per call by the
+		 * first host that renders, so no text written before the call can
+		 * realistically contain a placeholder.
 		 */
+		if ( '' === $this->html_placeholder_marker ) {
+			$this->html_placeholder_marker = 'wp-interactivity-dangerous-html-' . wp_generate_uuid4();
+		}
 		$placeholder = '<!--' . $this->html_placeholder_marker . ':' . count( $this->html_placeholders ) . '-->';
 		if ( $p->skip_to_balanced_tag_closer( $placeholder ) ) {
 			$this->html_placeholders[ $placeholder ] = $html;
diff --git a/tests/phpunit/tests/interactivity-api/wpInteractivityAPI-wp-html.php b/tests/phpunit/tests/interactivity-api/wpInteractivityAPI-wp-html.php
index dee43176fa721..5223808a71ebf 100644
--- a/tests/phpunit/tests/interactivity-api/wpInteractivityAPI-wp-html.php
+++ b/tests/phpunit/tests/interactivity-api/wpInteractivityAPI-wp-html.php
@@ -1093,15 +1093,17 @@ public function test_host_inside_a_rendering_host_is_replaced_with_it() {
 	}
 
 	/**
-	 * Tests that markup in the input that looks like a placeholder is not
-	 * mistaken for one, however many marker variants it holds.
+	 * Tests that markup in the input that looks like a placeholder, but holds a
+	 * marker other than the call's own, is not mistaken for one. That includes
+	 * the form of the call's own placeholders with a UUID other than the call's.
 	 *
 	 * @ticket 66246
 	 *
 	 * @covers ::process_directives
 	 */
 	public function test_substitution_cannot_be_forged() {
-		$lookalikes = '<!--wp-interactivity-dangerous-html:0--><!--wp-interactivity-dangerous-html_:0-->';
+		$lookalikes = '<!--wp-interactivity-dangerous-html:0--><!--wp-interactivity-dangerous-html_:0-->' .
+			'<!--wp-interactivity-dangerous-html-3f2b8c1e-7d4a-4e9b-a5c6-0d1e2f3a4b5c:0-->';
 		$this->set_state( array( 'html' => $this->token() ) );
 
 		$output = wp_interactivity_process_directives(
@@ -1114,33 +1116,6 @@ public function test_substitution_cannot_be_forged() {
 		);
 	}
 
-	/**
-	 * Tests that HTML that looks like the placeholder of another host is
-	 * inserted as given, not replaced by that host's HTML.
-	 *
-	 * @ticket 66246
-	 *
-	 * @covers ::process_directives
-	 */
-	public function test_inserted_html_is_not_searched_for_placeholders() {
-		$lookalike = '<!--wp-interactivity-dangerous-html:1-->';
-		$this->set_state(
-			array(
-				'first'  => wp_interactivity_as_dangerous_html( $lookalike ),
-				'second' => wp_interactivity_as_dangerous_html( '<u>second</u>' ),
-			)
-		);
-
-		$output = wp_interactivity_process_directives(
-			'<div data-wp-interactive="test"><p data-wp-html="state.first">One</p><p data-wp-html="state.second">Two</p></div>'
-		);
-
-		$this->assertSame(
-			'<div data-wp-interactive="test"><p data-wp-html="state.first">' . $lookalike . '</p><p data-wp-html="state.second"><u>second</u></p></div>',
-			$output
-		);
-	}
-
 	/**
 	 * Tests that unbalanced markup returns the input unchanged, with no HTML
 	 * inserted.