diff --git a/src/wp-includes/bits.php b/src/wp-includes/bits.php
new file mode 100644
index 0000000000000..a4edd2dea0dc1
--- /dev/null
+++ b/src/wp-includes/bits.php
@@ -0,0 +1,125 @@
+<?php
+/**
+ * WordPress Bits system, for replacing placeholder tokens in a document,
+ * for semantically independent content types, with externally-sourced data.
+ *
+ * @package    WordPress
+ * @subpackage Bits
+ * @since      {WP_VERSION}
+ */
+
+declare( strict_types=1 );
+
+/**
+ * Hello Dolly Bit Rendering function.
+ *
+ * @param string     $name        Fully-qualified name of the matched Bit.
+ *                                E.g. "core/post-author".
+ * @param string     $output_type Either "rich-text" or "plaintext" depending on where the Bit is found.
+ *                                E.g. when inside an HTML attribute or TITLE element, only plaintext is allowed,
+ *                                But when found inside a P element, rich formatting is allowed.
+ * @param array|null $attributes  Configured parameters of the Bit, if provided.
+ *                                E.g. `<//wp-bit:hello-dolly year="2024">` produces `array( 'year' => '2024' )`.
+ * @param mixed      $context     Context passed into the Bit from the surrounding system.
+ *                                This argument is not yet specified and will always be `null`.
+ *
+ * @return mixed An HTML template for rendering into the page, either as a plain string or in array form.
+ */
+function core_bit_hello_dolly( string $name, string $output_type, ?array $attributes, mixed $context ): mixed {
+	static $vocalists = array(
+		'Julie Dahle Aagård',
+		'Mindi Abair',
+		'Lorez Alexandria',
+		'Karrin Allyson',
+		'Michelle Amato',
+		'Ernestine Anderson',
+		'Ivie Anderson',
+	);
+
+	$vocalist = $vocalists[ wp_rand( 0, count( $vocalists ) - 1 ) ];
+
+	switch ( $output_type ) {
+		case 'plaintext':
+			return $vocalist;
+
+		case 'rich-text':
+			return array(
+				'<span data-vocalist="</%name>"></%name></span>',
+				array( 'name' => $vocalist ),
+			);
+	}
+}
+
+class WP_Hello_Dolly_Bit extends BitProvider {
+	/**
+	 * @inheritDoc
+	 */
+	public function handle_plaintext( string $bit_name, ?array $attributes ): string {
+		return self::get_random_vocalist();
+	}
+
+	/**
+	 * @inheritDoc
+	 */
+	public function handle_richtext( string $bit_name, ?array $attributes ): WP_HTML_Template {
+		$name = self::get_random_vocalist();
+
+		return new WP_HTML_Template(
+			'<span data-vocalist="</%name>"></%name></span>',
+			array( 'name' => $name )
+		);
+	}
+
+	/**
+	 * Returns the name of a random Jazz vocalist.
+	 *
+	 * @return string
+	 */
+	private function get_random_vocalist(): string {
+		static $vocalists = array(
+			'Julie Dahle Aagård',
+			'Mindi Abair',
+			'Lorez Alexandria',
+			'Karrin Allyson',
+			'Michelle Amato',
+			'Ernestine Anderson',
+			'Ivie Anderson',
+		);
+
+		return $vocalists[ wp_rand( 0, count( $vocalists ) - 1 ) ];
+	}
+}
+
+abstract class BitProvider {
+	/**
+	 * Performs initialize of Bit Provider during WordPress bootup.
+	 */
+	public function register(): void {
+		// This is optional.
+	};
+
+	/**
+	 * Called to source content in plaintext contexts. For example, when a
+	 * Bit is found within an HTML attribute, or inside a `TITLE` element.
+	 *
+	 * @see WP_HTML_Template
+	 *
+	 * @param string     $bit_name   Full name with namespace of matched Bit, e.g. "core/post-author".
+	 * @param array|null $attributes Configured attributes found on Bit, if found, otherwise `null`.
+	 *
+	 * @return string Plaintext value for provided content.
+	 */
+	abstract public function handle_plaintext( string $bit_name, ?array $attributes ): string;
+
+	/**
+	 * Called to source content in Rich Text (HTML Markup) contexts. For example,
+	 * when a Bit is found within the inner content of an HTML tag.
+	 *
+	 * @see WP_HTML_Template
+	 *
+	 * @param string     $bit_name   Full name with namespace of matched Bit, e.g. "core/post-author".
+	 * @param array|null $attributes Configured attributes found on Bit, if found, otherwise `null`.
+	 * @return WP_HTML_Template HTML template for provided content: a string or array.
+	 */
+	abstract public function handle_richtext( string $bit_name, ?array $attributes ): WP_HTML_Template;
+}
diff --git a/src/wp-includes/blocks.php b/src/wp-includes/blocks.php
index decc4896c378a..032e08ffc7502 100644
--- a/src/wp-includes/blocks.php
+++ b/src/wp-includes/blocks.php
@@ -2057,6 +2057,8 @@ function render_block( $parsed_block ) {
  * }
  */
 function parse_blocks( $content ) {
+	$content = wp_replace_bits( $content );
+
 	/**
 	 * Filter to allow plugins to replace the server-side block parser.
 	 *
diff --git a/src/wp-includes/html-api/class-wp-bits.php b/src/wp-includes/html-api/class-wp-bits.php
new file mode 100644
index 0000000000000..a7652bf64da1c
--- /dev/null
+++ b/src/wp-includes/html-api/class-wp-bits.php
@@ -0,0 +1,77 @@
+<?php
+
+function wp_replace_bits( $content ) {
+	$processor = new class ( $content ) extends WP_HTML_Tag_Processor {
+		private $deferred_updates = array();
+
+		public function replace_token( $new_content ) {
+			$this->set_bookmark( 'here' );
+			$here = $this->bookmarks['here'];
+
+			$this->deferred_updates[] = new WP_HTML_Text_Replacement(
+				$here->start,
+				$here->length,
+				$new_content
+			);
+		}
+
+		public function flush_updates() {
+			foreach ( $this->deferred_updates as $update ) {
+				$this->lexical_updates[] = $update;
+			}
+		}
+	};
+
+	while ( $processor->next_token() ) {
+		switch ( $processor->get_token_type() ) {
+			case '#funky-comment':
+				$processor->replace_token( '<b>bl<em>ar</em>g</b>' );
+				break;
+
+			case '#tag':
+				foreach ( $processor->get_attribute_names_with_prefix( '' ) ?? [] as $name ) {
+					$value = $processor->get_attribute( $name );
+					if ( is_string( $value ) ) {
+						$new_value = preg_replace_callback(
+							'~<//wp:([^>]+)>~',
+							static function ( $bit ) {
+								return 'blarg';
+							},
+							$value
+						);
+
+						if ( $new_value !== $value ) {
+							$processor->set_attribute( $name, $new_value );
+						}
+					}
+				}
+				break;
+
+			case '#comment':
+				if ( WP_HTML_Tag_Processor::COMMENT_AS_HTML_COMMENT !== $processor->get_comment_type() ) {
+					break;
+				}
+
+				$text = $processor->get_modifiable_text();
+				if ( 1 === preg_match( '~^<//wp:([^>]+)>$~', $text ) ) {
+					$processor->replace_token( '<b>Bla<em>rg</em>!</b>' );
+					break;
+				}
+
+				$new_value = preg_replace_callback(
+					'~<//wp:([^>]+)>~',
+					static function ( $bit ) {
+						return 'blarg';
+					},
+					$text
+				);
+
+				$processor->replace_token( "<!--{$new_value}-->" );
+				break;
+		}
+	}
+	$processor->flush_updates();
+	$content = $processor->get_updated_html();
+
+	return $content;
+}
diff --git a/src/wp-includes/html-api/class-wp-html-template.php b/src/wp-includes/html-api/class-wp-html-template.php
new file mode 100644
index 0000000000000..f181478e41e63
--- /dev/null
+++ b/src/wp-includes/html-api/class-wp-html-template.php
@@ -0,0 +1,48 @@
+<?php
+
+declare( strict_types=1 );
+
+class WP_HTML_Template {
+	/**
+	 * HTML Template indicating where to mix a static HTML template
+	 * and placeholders for dynamic values.
+	 *
+	 * @var string
+	 */
+	public $template;
+
+	/**
+	 * Stores data necessary to render a template, if any required.
+	 *
+	 * @var array|null
+	 */
+	public $data;
+
+	/**
+	 * Constructor function.
+	 *
+	 * Example:
+	 *
+	 *     // No placeholders are required, only a template string.
+	 *     new WP_HTML_Template( '<p>Hello, World!</p>' );
+	 *
+	 *     // Placeholders for simple substitution.
+	 *     new WP_HTML_Template( '<p>Hello, </%name>!</p>', array( 'name' => $name ) );
+	 *
+	 *     // Spread-operator for sets of attributes.
+	 *     new WP_HTML_Template(
+	 *         '<button ...interactivity_args>Click me!</button>',
+	 *         array(
+	 *             'data-wp-text="context.buttonLabel"',
+	 *             'data-wp-click="actions.clickButton",
+	 *         )
+	 *     );
+	 *
+	 * @param string     $template Static HTML template, possibly including placeholders.
+	 * @param array|null $data     Optional. Data provided for placeholders, if any.
+	 */
+	public function __construct( string $template, array $data = null ) {
+		$this->template = $template;
+		$this->data     = $data;
+	}
+}
diff --git a/src/wp-settings.php b/src/wp-settings.php
index a682c2a50380a..2071922cd2da0 100644
--- a/src/wp-settings.php
+++ b/src/wp-settings.php
@@ -262,6 +262,8 @@
 require ABSPATH . WPINC . '/html-api/class-wp-html-stack-event.php';
 require ABSPATH . WPINC . '/html-api/class-wp-html-processor-state.php';
 require ABSPATH . WPINC . '/html-api/class-wp-html-processor.php';
+require ABSPATH . WPINC . '/html-api/class-wp-html-template.php';
+require ABSPATH . WPINC . '/html-api/class-wp-bits.php';
 require ABSPATH . WPINC . '/class-wp-http.php';
 require ABSPATH . WPINC . '/class-wp-http-streams.php';
 require ABSPATH . WPINC . '/class-wp-http-curl.php';