diff --git a/phpunit/directives/wp-directive-processor.php b/phpunit/directives/wp-directive-processor.php
new file mode 100644
index 00000000..8ca135de
--- /dev/null
+++ b/phpunit/directives/wp-directive-processor.php
@@ -0,0 +1,117 @@
+<?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_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() {
+		$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..324df9f5
--- /dev/null
+++ b/src/directives/class-wp-directive-processor.php
@@ -0,0 +1,137 @@
+<?php
+
+class WP_Directive_Processor extends WP_HTML_Tag_Processor {
+	const DIRECTIVE_PREFIX = 'WPX-';
+
+	public $html;
+
+	public $might_have_directives = true;
+
+	public function __construct( $html ) {
+		parent::__construct( $html );
+
+		if ( false === stripos( self::DIRECTIVE_PREFIX, $html ) ) {
+			$this->might_have_directives = false;
+		}
+	}
+
+	public function next_directive() {
+		if ( false === $this->might_have_directives ) {
+			return false;
+		}
+
+		while ( $this->next_tag( array( 'tag_closers' => 'visit' ) ) ) {
+			$tag_name = $this->get_tag();
+			if ( 0 === stripos( self::DIRECTIVE_PREFIX, $tag_name ) ) {
+				return true;
+			}
+
+			$attribute_directives = $this->get_attribute_names_with_prefix( self::DIRECTIVE_PREFIX );
+			if ( 0 < count( $attribute_directives ) ) {
+				return true;
+			}
+		}
+
+		return false;
+	}
+
+	public function next_balanced_closer() {
+		$depth = 0;
+
+		$tag_name = $this->get_tag();
+		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;
+	}
+
+	public function get_inner_html() {
+		$this->set_bookmark( 'start' );
+		if ( ! $this->next_balanced_closer() ) {
+			$this->release_bookmark( 'start' );
+			return false;
+		}
+		$this->set_bookmark( 'end' );
+
+		$start = $this->bookmarks['start']->end + 1;
+		$end   = $this->bookmarks['end']->start;
+
+		$this->release_bookmark( 'start' );
+		$this->release_bookmark( 'end' );
+
+		return substr( $this->html, $start, $end - $start );
+	}
+
+	public function set_inner_html( $new_html ) {
+		$this->set_bookmark( 'start' );
+		if ( ! $this->next_balanced_closer() ) {
+			$this->release_bookmark( 'start' );
+			return false;
+		}
+		$this->set_bookmark( 'end' );
+
+		$start = $this->bookmarks['start']->end + 1;
+		$end   = $this->bookmarks['end']->start;
+
+		$this->release_bookmark( 'start' );
+		$this->release_bookmark( 'end' );
+
+		$this->lexical_updates[] = new WP_HTML_Text_Replacement( $start, $end, $new_html );
+		return true;
+	}
+
+	public function get_outer_html( $new_html ) {
+		$this->set_bookmark( 'start' );
+		if ( ! $this->next_balanced_closer() ) {
+			$this->release_bookmark( 'start' );
+			return false;
+		}
+		$this->set_bookmark( 'end' );
+
+		$start = $this->bookmarks['start']->start;
+		$end   = $this->bookmarks['end']->end + 1;
+
+		$this->release_bookmark( 'start' );
+		$this->release_bookmark( 'end' );
+
+		// For consistency with set_outer_html:
+		$this->next_tag();
+		return substr( $this->html, $start, $end - $start );
+	}
+
+	public function set_outer_html( $new_html ) {
+		$this->set_bookmark( 'start' );
+		if ( ! $this->next_balanced_closer() ) {
+			$this->release_bookmark( 'start' );
+			return false;
+		}
+		$this->set_bookmark( 'end' );
+
+		$start = $this->bookmarks['start']->start;
+		$end   = $this->bookmarks['end']->end + 1;
+
+		$this->release_bookmark( 'start' );
+		$this->release_bookmark( 'end' );
+
+		$this->next_tag();
+		$this->set_bookmark( 'next' );
+		$this->lexical_updates[] = new WP_HTML_Text_Replacement( $start, $end, $new_html );
+		// updates before the current position are not supported well and we end
+		// up at an invalid combination of copied bytes and parsed bytes index. 
+		// bookmarks are updated correctly, though, so seek() makes it right again.
+		$this->seek('next');
+		$this->release_bookmark( 'next' );
+		return true;
+	}
+	
+}