diff --git a/phpunit/directives/wp-directive-processor.php b/phpunit/directives/wp-directive-processor.php
new file mode 100644
index 00000000..e24e247c
--- /dev/null
+++ b/phpunit/directives/wp-directive-processor.php
@@ -0,0 +1,128 @@
+<?php
+/**
+ * `WP_Directive_Processor` class test.
+ */
+require_once __DIR__ . '/../../src/directives/class-wp-directive-processor.php';
+
+/**
+ * @group html-processor
+ *
+ * @coversDefaultClass WP_Directive_Processor
+ */
+class WP_Directive_Processor_Test extends WP_UnitTestCase {
+	const HTML = '<div>outside</div><section><div><img>inside</div></section>';
+
+	public function test_next_balanced_closer_stays_on_void_tag() {
+		$tags = new WP_Directive_Processor( self::HTML );
+
+		$tags->next_tag( 'img' );
+		$result = $tags->next_balanced_closer();
+		$this->assertSame( 'IMG', $tags->get_tag() );
+		$this->assertFalse( $result );
+	}
+
+	public function test_next_balanced_closer_proceeds_to_correct_tag() {
+		$tags = new WP_Directive_Processor( self::HTML );
+
+		$tags->next_tag( 'section' );
+		$tags->next_balanced_closer();
+		$this->assertSame( 'SECTION', $tags->get_tag() );
+		$this->assertTrue( $tags->is_tag_closer() );
+	}
+
+	public function test_next_balanced_closer_proceeds_to_correct_tag_for_nested_tag() {
+		$tags = new WP_Directive_Processor( self::HTML );
+
+		$tags->next_tag( 'div' );
+		$tags->next_tag( 'div' );
+		$tags->next_balanced_closer();
+		$this->assertSame( 'DIV', $tags->get_tag() );
+		$this->assertTrue( $tags->is_tag_closer() );
+	}
+
+	public function test_get_inner_html_returns_correct_result() {
+		$tags = new WP_Directive_Processor( self::HTML );
+
+		$tags->next_tag( 'section' );
+		$this->assertSame( '<div><img>inside</div>', $tags->get_inner_html() );
+	}
+
+	public function test_set_inner_html_on_void_element_has_no_effect() {
+		$tags = new WP_Directive_Processor( self::HTML );
+
+		$tags->next_tag( 'img' );
+		$content = $tags->set_inner_html( 'This is the new img content' );
+		$this->assertFalse( $content );
+		$this->assertSame( self::HTML, $tags->get_updated_html() );
+	}
+
+	public function test_set_inner_html_sets_content_correctly() {
+		$tags = new WP_Directive_Processor( self::HTML );
+
+		$tags->next_tag( 'section' );
+		$tags->set_inner_html( 'This is the new section content.' );
+		$this->assertSame( '<div>outside</div><section>This is the new section content.</section>', $tags->get_updated_html() );
+	}
+
+	public function test_set_inner_html_updates_bookmarks_correctly() {
+		$tags = new WP_Directive_Processor( self::HTML );
+
+		$tags->next_tag( 'div' );
+		$tags->set_bookmark( 'start' );
+		$tags->next_tag( 'img' );
+		$this->assertSame( 'IMG', $tags->get_tag() );
+		$tags->set_bookmark( 'after' );
+		$tags->seek( 'start' );
+
+		$tags->set_inner_html( 'This is the new div content.' );
+		$this->assertSame( '<div>This is the new div content.</div><section><div><img>inside</div></section>', $tags->get_updated_html() );
+		$tags->seek( 'after' );
+		$this->assertSame( 'IMG', $tags->get_tag() );
+	}
+
+	public function test_set_inner_html_subsequent_updates_on_the_same_tag_work() {
+		$tags = new WP_Directive_Processor( self::HTML );
+
+		$tags->next_tag( 'section' );
+		$tags->set_inner_html( 'This is the new section content.' );
+		$tags->set_inner_html( 'This is the even newer section content.' );
+		$this->assertSame( '<div>outside</div><section>This is the even newer section content.</section>', $tags->get_updated_html() );
+	}
+
+	public function test_set_inner_html_followed_by_set_attribute_works() {
+		$tags = new WP_Directive_Processor( self::HTML );
+
+		$tags->next_tag( 'section' );
+		$tags->set_inner_html( 'This is the new section content.' );
+		$tags->set_attribute( 'id', 'thesection' );
+		$this->assertSame( '<div>outside</div><section id="thesection">This is the new section content.</section>', $tags->get_updated_html() );
+	}
+
+	public function test_set_inner_html_preceded_by_set_attribute_works() {
+		$tags = new WP_Directive_Processor( self::HTML );
+
+		$tags->next_tag( 'section' );
+		$tags->set_attribute( 'id', 'thesection' );
+		$tags->set_inner_html( 'This is the new section content.' );
+		$this->assertSame( '<div>outside</div><section id="thesection">This is the new section content.</section>', $tags->get_updated_html() );
+	}
+
+	public function test_set_inner_html_invalidates_bookmarks_that_point_to_replaced_content() {
+		$this->markTestSkipped( "This requires on bookmark invalidation, which is only in GB's WP 6.3 compat layer." );
+
+		$tags = new WP_Directive_Processor( self::HTML );
+
+		$tags->next_tag( 'section' );
+		$tags->set_bookmark( 'start' );
+		$tags->next_tag( 'img' );
+		$tags->set_bookmark( 'replaced' );
+		$tags->seek( 'start' );
+
+		$tags->set_inner_html( 'This is the new section content.' );
+		$this->assertSame( '<div>outside</div><section>This is the new section content.</section>', $tags->get_updated_html() );
+
+		$this->expectExceptionMessage( 'Invalid bookmark name' );
+		$successful_seek = $tags->seek( 'replaced' );
+		$this->assertFalse( $successful_seek );
+	}
+}
diff --git a/src/directives/class-wp-directive-processor.php b/src/directives/class-wp-directive-processor.php
new file mode 100644
index 00000000..d9a18cf0
--- /dev/null
+++ b/src/directives/class-wp-directive-processor.php
@@ -0,0 +1,155 @@
+<?php
+
+class WP_Directive_Processor extends WP_HTML_Tag_Processor {
+	/**
+	 * Find the matching closing tag for an opening tag.
+	 *
+	 * When called while on an open tag, traverse the HTML until we find
+	 * the matching closing tag, respecting any in-between content, including
+	 * nested tags of the same name. Return false when called on a closing or
+	 * void tag, or if no matching closing tag was found.
+	 *
+	 * @return bool True if a matching closing tag was found.
+	 */
+	public function next_balanced_closer() {
+		$depth = 0;
+
+		$tag_name = $this->get_tag();
+
+		if ( self::is_html_void_element( $tag_name ) ) {
+			return false;
+		}
+
+		while ( $this->next_tag(
+			array(
+				'tag_name'    => $tag_name,
+				'tag_closers' => 'visit',
+			)
+		) ) {
+			if ( ! $this->is_tag_closer() ) {
+				$depth++;
+				continue;
+			}
+
+			if ( 0 === $depth ) {
+				return true;
+			}
+
+			$depth--;
+		}
+
+		return false;
+	}
+
+	/**
+	 * Return the content between two balanced tags.
+	 *
+	 * When called on an opening tag, return the HTML content found between
+	 * that opening tag and its matching closing tag.
+	 *
+	 * @return string The content between the current opening and its matching closing tag.
+	 */
+	public function get_inner_html() {
+		$bookmarks = $this->get_balanced_tag_bookmarks();
+		if ( ! $bookmarks ) {
+			return false;
+		}
+		list( $start_name, $end_name ) = $bookmarks;
+
+		$start = $this->bookmarks[ $start_name ]->end + 1;
+		$end   = $this->bookmarks[ $end_name ]->start;
+
+		$this->seek( $start_name ); // Return to original position.
+		$this->release_bookmark( $start_name );
+		$this->release_bookmark( $end_name );
+
+		return substr( $this->html, $start, $end - $start );
+	}
+
+	/**
+	 * Set the content between two balanced tags.
+	 *
+	 * When called on an opening tag, set the HTML content found between
+	 * that opening tag and its matching closing tag.
+	 *
+	 * @param string $new_html The string to replace the content between the matching tags with.
+	 * @return bool            Whether the content was successfully replaced.
+	 */
+	public function set_inner_html( $new_html ) {
+		$this->get_updated_html(); // Apply potential previous updates.
+
+		$bookmarks = $this->get_balanced_tag_bookmarks();
+		if ( ! $bookmarks ) {
+			return false;
+		}
+		list( $start_name, $end_name ) = $bookmarks;
+
+		$start = $this->bookmarks[ $start_name ]->end + 1;
+		$end   = $this->bookmarks[ $end_name ]->start;
+
+		$this->seek( $start_name ); // Return to original position.
+		$this->release_bookmark( $start_name );
+		$this->release_bookmark( $end_name );
+
+		$this->lexical_updates[] = new WP_HTML_Text_Replacement( $start, $end, $new_html );
+		return true;
+	}
+
+	/**
+	 * Return a pair of bookmarks for the current opening tag and the matching closing tag.
+	 *
+	 * @return array|false A pair of bookmarks, or false if there's no matching closing tag.
+	 */
+	public function get_balanced_tag_bookmarks() {
+		$i = 0;
+		while ( array_key_exists( 'start' . $i, $this->bookmarks ) ) {
+			++$i;
+		}
+		$start_name = 'start' . $i;
+
+		$this->set_bookmark( $start_name );
+		if ( ! $this->next_balanced_closer() ) {
+			$this->release_bookmark( $start_name );
+			return false;
+		}
+
+		$i = 0;
+		while ( array_key_exists( 'end' . $i, $this->bookmarks ) ) {
+			++$i;
+		}
+		$end_name = 'end' . $i;
+		$this->set_bookmark( $end_name );
+
+		return array( $start_name, $end_name );
+	}
+
+	/**
+	 * Whether a given HTML element is void (e.g. <br>).
+	 *
+	 * @param string $tag_name The element in question.
+	 * @return bool True if the element is void.
+	 *
+	 * @see https://html.spec.whatwg.org/#elements-2
+	 */
+	public static function is_html_void_element( $tag_name ) {
+		switch ( $tag_name ) {
+			case 'AREA':
+			case 'BASE':
+			case 'BR':
+			case 'COL':
+			case 'EMBED':
+			case 'HR':
+			case 'IMG':
+			case 'INPUT':
+			case 'LINK':
+			case 'META':
+			case 'SOURCE':
+			case 'TRACK':
+			case 'WBR':
+				return true;
+
+			default:
+				return false;
+		}
+	}
+}
diff --git a/src/directives/wp-process-directives.php b/src/directives/wp-process-directives.php
index c465d6ba..21ca430a 100644
--- a/src/directives/wp-process-directives.php
+++ b/src/directives/wp-process-directives.php
@@ -1,6 +1,7 @@
 <?php
 
 require_once __DIR__ . '/class-wp-directive-context.php';
+require_once __DIR__ . '/class-wp-directive-processor.php';
 
 function wp_process_directives( $tags, $prefix, $directives ) {
 	$context = new WP_Directive_Context;
@@ -41,7 +42,7 @@ function wp_process_directives( $tags, $prefix, $directives ) {
 			// directives so we can call its directive processor once we encounter the
 			// matching closing tag.
 			if (
-				! is_html_void_element( $tags->get_tag() ) &&
+				! WP_Directive_Processor::is_html_void_element( $tags->get_tag() ) &&
 				( 0 !== count( $attributes ) || 0 !== count( $tag_stack ) )
 			) {
 				$tag_stack[] = array( $tag_name, $attributes );
@@ -56,26 +57,3 @@ function wp_process_directives( $tags, $prefix, $directives ) {
 	return $tags;
 }
 
-// TODO: Move into `WP_HTML_Tag_Processor` (or `WP_HTML_Processor`).
-// See e.g. https://github.com/WordPress/gutenberg/pull/47573.
-function is_html_void_element( $tag_name ) {
-	switch ( $tag_name ) {
-		case 'AREA':
-		case 'BASE':
-		case 'BR':
-		case 'COL':
-		case 'EMBED':
-		case 'HR':
-		case 'IMG':
-		case 'INPUT':
-		case 'LINK':
-		case 'META':
-		case 'SOURCE':
-		case 'TRACK':
-		case 'WBR':
-			return true;
-
-		default:
-			return false;
-	}
-}