diff --git a/src/wp-includes/html-api/class-wp-html-processor.php b/src/wp-includes/html-api/class-wp-html-processor.php
index 973effd7bff1b..60d7849be5f94 100644
--- a/src/wp-includes/html-api/class-wp-html-processor.php
+++ b/src/wp-includes/html-api/class-wp-html-processor.php
@@ -442,20 +442,20 @@ public function get_inner_markup() {
 			return null;
 		}
 
-		$this->set_bookmark( 'start' );
+		$this->set_bookmark( 'opener' );
 		$found_tag = $this->step_until_tag_is_closed();
-		$this->set_bookmark( 'end' );
+		$this->set_bookmark( 'closer' );
 
 		if ( $found_tag ) {
-			$inner_markup = $this->substr_bookmarks( 'after', 'start', 'before', 'end' );
+			$inner_markup = $this->substr_bookmarks( 'after', 'opener', 'before', 'closer' );
 		} else {
 			// If there's no closing tag then the inner markup continues to the end of the document.
-			$inner_markup = $this->substr_bookmark( 'after', 'start' );
+			$inner_markup = $this->substr_bookmark( 'after', 'opener' );
 		}
 
-		$this->seek( 'start' );
-		$this->release_bookmark( 'start' );
-		$this->release_bookmark( 'end' );
+		$this->seek( 'opener' );
+		$this->release_bookmark( 'opener' );
+		$this->release_bookmark( 'closer' );
 
 		return $inner_markup;
 	}
@@ -484,31 +484,75 @@ public function get_outer_markup() {
 			return null;
 		}
 
-		$this->set_bookmark( 'start' );
+		$this->set_bookmark( 'opener' );
 		$start_tag = $this->current_token->node_name;
 		$found_tag = $this->step_until_tag_is_closed();
-		$this->set_bookmark( 'end' );
+		$this->set_bookmark( 'closer' );
 
 		if ( $found_tag ) {
 			$did_close    = $this->get_tag() === $start_tag && $this->is_tag_closer();
 			$end_position = $did_close ? 'after' : 'before';
-			$outer_markup = $this->substr_bookmarks( 'before', 'start', $end_position, 'end' );
+			$outer_markup = $this->substr_bookmarks( 'before', 'opener', $end_position, 'closer' );
 		} else {
 			// If there's no closing tag then the outer markup continues to the end of the document.
-			$outer_markup = $this->substr_bookmark( 'before', 'start' );
+			$outer_markup = $this->substr_bookmark( 'before', 'opener' );
 		}
 
-		$this->seek( 'start' );
-		$this->release_bookmark( 'start' );
-		$this->release_bookmark( 'end' );
+		$this->seek( 'opener' );
+		$this->release_bookmark( 'opener' );
+		$this->release_bookmark( 'closer' );
 
 		return $outer_markup;
 	}
 
+	/**
+	 * Replaces the raw HTML of the currently-matched tag's inner markup with new HTML.
+	 * This replaces the content between the tag opener and tag closer.
+	 *
+	 * @throws Exception When unable to set bookmark for internal tracking.
+	 *
+	 * @since 6.4.0
+	 *
+	 * @param string $new_html
+	 * @return bool|null Whether the contents were updated.
+	 */
+	public function set_inner_markup( $new_html ) {
+		if ( null === $this->get_tag() ) {
+			return null;
+		}
+
+		$this->set_bookmark( 'opener' );
+		$start_tag = $this->current_token->node_name;
+
+		if ( self::is_void( $start_tag ) ) {
+			$this->release_bookmark( 'opener' );
+			return true;
+		}
+
+		$found_tag = $this->step_until_tag_is_closed();
+		$this->set_bookmark( 'closer' );
+
+		if ( $found_tag ) {
+			$this->replace_using_bookmarks( $new_html, 'after', 'opener', 'before', 'closer' );
+		} else {
+			// If there's no closing tag then the inner markup continues to the end of the document.
+			$this->replace_using_bookmark( $new_html, 'after', 'opener' );
+		}
+
+		$this->seek( 'opener' );
+		$this->release_bookmark( 'opener' );
+		$this->release_bookmark( 'closer' );
+		return true;
+	}
+
 	/**
 	 * Replaces the raw HTML of the currently-matched tag with new HTML.
 	 * This replaces the entire contents of the tag including the tag itself.
 	 *
+	 * @throws Exception When unable to set bookmark for internal tracking.
+	 *
+	 * @since 6.4.0
+	 *
 	 * @param string $new_html
 	 * @return bool|null Whether the contents were updated.
 	 */
@@ -517,30 +561,30 @@ public function set_outer_markup( $new_html ) {
 			return null;
 		}
 
-		$this->set_bookmark( 'start' );
+		$this->set_bookmark( 'opener' );
 		$start_tag = $this->current_token->node_name;
 
 		if ( self::is_void( $start_tag ) ) {
-			$this->replace_using_bookmarks( $new_html, 'before', 'start', 'after', 'start' );
-			$this->release_bookmark( 'start' );
+			$this->replace_using_bookmarks( $new_html, 'before', 'opener', 'after', 'opener' );
+			$this->release_bookmark( 'opener' );
 			return true;
 		}
 
 		$found_tag = $this->step_until_tag_is_closed();
-		$this->set_bookmark( 'end' );
+		$this->set_bookmark( 'closer' );
 
 		if ( $found_tag ) {
 			$did_close    = $this->get_tag() === $start_tag && $this->is_tag_closer();
 			$end_position = $did_close ? 'after' : 'before';
-			$this->replace_using_bookmarks( $new_html, 'before', 'start', $end_position, 'end' );
+			$this->replace_using_bookmarks( $new_html, 'before', 'opener', $end_position, 'closer' );
 		} else {
 			// If there's no closing tag then the outer markup continues to the end of the document.
-			$this->replace_using_bookmark( $new_html, 'before', 'start' );
+			$this->replace_using_bookmark( $new_html, 'before', 'opener' );
 		}
 
-		$this->seek( 'start' );
-		$this->release_bookmark( 'start' );
-		$this->release_bookmark( 'end' );
+		$this->seek( 'opener' );
+		$this->release_bookmark( 'opener' );
+		$this->release_bookmark( 'closer' );
 		return true;
 	}
 
diff --git a/tests/phpunit/tests/html-api/wpHtmlProcessorSetInnerMarkup.php b/tests/phpunit/tests/html-api/wpHtmlProcessorSetInnerMarkup.php
index e69de29bb2d1d..20ec56a63e47c 100644
--- a/tests/phpunit/tests/html-api/wpHtmlProcessorSetInnerMarkup.php
+++ b/tests/phpunit/tests/html-api/wpHtmlProcessorSetInnerMarkup.php
@@ -0,0 +1,123 @@
+<?php
+/**
+ * Unit tests covering WP_HTML_Processor::set_inner_markup()
+ *
+ * @package WordPress
+ * @subpackage HTML-API
+ *
+ * @since 6.4.0
+ *
+ * @group html-api
+ *
+ * @coversDefaultClass WP_HTML_Processor
+ */
+class Tests_HtmlApi_WpHtmlProcessorSetInnerMarkup extends WP_UnitTestCase {
+	/**
+	 * @ticket {TICKET_NUMBER}
+	 *
+	 * @covers WP_HTML_Processor::set_inner_markup
+	 *
+	 * @dataProvider data_html_with_inner_markup_changes
+	 *
+	 * @since 6.4.0
+	 *
+	 * @param string $html_with_target_node HTML containing a node with the `target` attribute set.
+	 * @param string $new_markup            HTML for replacing the inner markup of the target node.
+	 * @param string $expected_output       New HTMl after replacing inner markup.
+	 */
+	public function test_replaces_inner_html_appropriately( $html_with_target_node, $new_markup, $expected_output ) {
+		$p = WP_HTML_Processor::createFragment( $html_with_target_node );
+
+		while ( $p->next_tag() && null === $p->get_attribute( 'target' ) ) {
+			continue;
+		}
+
+		$this->assertTrue( $p->set_inner_markup( $new_markup ), 'Failed to set inner markup.' );
+		$this->assertSame( $expected_output, $p->get_updated_html(), 'Failed to appropriately set inner markup.' );
+	}
+
+	/**
+	 * Data provider.
+	 *
+	 * @return array[]
+	 */
+	public function data_html_with_inner_markup_changes() {
+		$data = array(
+			'Void element'                        => array( '<img target>', '', '<img target>' ),
+			'Void element inside text'            => array( 'before<img src="atat.png" loading=lazy target>after', '', 'before<img src="atat.png" loading=lazy target>after' ),
+			'Void element inside another element' => array( '<p>Look at this <img target> graph.</p>', '', '<p>Look at this <img target> graph.</p>' ),
+			'Empty elements'                      => array( '<div target></div>', '', '<div target></div>' ),
+			'Element with nested tags'            => array( '<div target>inside <span>the</span> div</div>', '', '<div target></div>' ),
+			'Element inside another element'      => array( '<div>inside <span target>the</span> div</div>', '', '<div>inside <span target></span> div</div>' ),
+			'Unclosed element'                    => array( '<div target>This is <em>all</em> inside the DIV', '', '<div target>' ),
+			'Unclosed nested element'             => array( '<div><p target>One thought<p>And another', '', '<div><p target><p>And another' ),
+			'Partially-closed element'            => array( '<div target>This is <em>all</em> inside the DIV</div', '', '<div target>' ),
+			'Implicitly-closed element'           => array( '<div><p target>Inside the P</div>Outside the P</p>', '', '<div><p target></div>Outside the P</p>' ),
+		);
+
+		$inner_html = <<<HTML
+			<p>This is inside the <strong>Match</strong></p>
+			<p><img></p>
+			<div>
+				  <figure>
+					   <img>
+					   <figcaption>Look at the <strike>picture</strike> photograph.</figcaption>
+				  </figure>
+			</div>
+HTML;
+
+		$prefix = <<<HTML
+			<div>
+				<p>This is not in the match.
+				<p>This is another paragraph not <a href="#">in</a> the match.
+			</div>
+			<div target>
+HTML;
+
+		/*
+		 * Removing the indent on this first line keeps the test easy to reason about,
+		 * otherwise extra indents appear after removing the inner content, because
+		 * that indentation before and after is whitespace and not part of the tag.
+		 */
+		$suffix = <<<HTML
+</div>
+			<div>
+				<p>This is also note in the match.</p>
+			</div>
+HTML;
+
+		$data['Complicated inner nesting'] = array( $prefix . $inner_html . $suffix, '', $prefix . $suffix );
+
+		return $data;
+	}
+
+	/**
+	 * Ensures that the cursor isn't moved when setting the inner markup. It should
+	 * remain at the same location as the tag opener where it was called.
+	 *
+	 * @ticket {TICKET_NUMBER}
+	 *
+	 * @covers WP_HTML_Processor::set_inner_markup
+	 *
+	 * @since 6.4.0
+	 */
+	public function test_preserves_cursor() {
+		$p = WP_HTML_Processor::createFragment( '<div><p><span>The <code target>cursor</code> should not move <em next-target>unexpectedly</em>.</span></p></div>' );
+
+		while ( $p->next_tag() && null === $p->get_attribute( 'target' ) ) {
+			continue;
+		}
+
+		$this->assertTrue( $p->set_inner_markup( '<img next-target>' ) );
+		$this->assertSame(
+			'<div><p><span>The <code target><img next-target></code> should not move <em next-target>unexpectedly</em>.</span></p></div>',
+			$p->get_updated_html(),
+			'Failed to replace appropriate inner markup.'
+		);
+
+		$this->assertSame( 'CODE', $p->get_tag(), "Should have remained on CODE, but found {$p->get_tag()} instead." );
+
+		$p->next_tag();
+		$this->assertNotNull( $p->get_attribute( 'next-target' ), "Expected to move to inserted IMG element, but found {$p->get_tag()} instead." );
+	}
+}