diff --git a/autoload.php b/autoload.php
index e0e1ba3f842..eb2fcaf38bc 100644
--- a/autoload.php
+++ b/autoload.php
@@ -1684,6 +1684,8 @@ $wgAutoloadLocalClasses = [
 	'MediaWiki\\Rest\\Handler\\CreationHandler' => __DIR__ . '/includes/Rest/Handler/CreationHandler.php',
 	'MediaWiki\\Rest\\Handler\\EditHandler' => __DIR__ . '/includes/Rest/Handler/EditHandler.php',
 	'MediaWiki\\Rest\\Handler\\HtmlInputTransformHelper' => __DIR__ . '/includes/Rest/Handler/HtmlInputTransformHelper.php',
+	'MediaWiki\\Rest\\Handler\\HtmlMessageOutputHelper' => __DIR__ . '/includes/Rest/Handler/HtmlMessageOutputHelper.php',
+	'MediaWiki\\Rest\\Handler\\HtmlOutputHelper' => __DIR__ . '/includes/Rest/Handler/HtmlOutputHelper.php',
 	'MediaWiki\\Rest\\Handler\\HtmlOutputRendererHelper' => __DIR__ . '/includes/Rest/Handler/HtmlOutputRendererHelper.php',
 	'MediaWiki\\Rest\\Handler\\LanguageLinksHandler' => __DIR__ . '/includes/Rest/Handler/LanguageLinksHandler.php',
 	'MediaWiki\\Rest\\Handler\\MediaFileHandler' => __DIR__ . '/includes/Rest/Handler/MediaFileHandler.php',
diff --git a/includes/Rest/Handler/HtmlMessageOutputHelper.php b/includes/Rest/Handler/HtmlMessageOutputHelper.php
new file mode 100644
index 00000000000..997de56e9fd
--- /dev/null
+++ b/includes/Rest/Handler/HtmlMessageOutputHelper.php
@@ -0,0 +1,123 @@
+<?php
+/**
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc.,
+ * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
+ * http://www.gnu.org/copyleft/gpl.html
+ *
+ * @file
+ */
+namespace MediaWiki\Rest\Handler;
+
+use LanguageCode;
+use MediaWiki\Page\PageIdentity;
+use MediaWiki\Rest\ResponseInterface;
+use Message;
+use ParserOutput;
+use Title;
+use Wikimedia\Parsoid\Utils\ContentUtils;
+use Wikimedia\Parsoid\Utils\DOMUtils;
+
+/**
+ * @since 1.40
+ * @unstable
+ */
+class HtmlMessageOutputHelper implements HtmlOutputHelper {
+
+	private PageIdentity $page;
+
+	/**
+	 * Initializes the helper with the given parameters like the page
+	 * we're dealing with.
+	 *
+	 * @param PageIdentity $page
+	 */
+	public function init( PageIdentity $page ): void {
+		$this->page = $page;
+	}
+
+	/**
+	 * @return Message|null
+	 */
+	private function getDefaultSystemMessage(): ?Message {
+		$title = Title::castFromPageIdentity( $this->page );
+
+		return $title ? $title->getDefaultSystemMessage() : null;
+	}
+
+	/**
+	 * @inheritDoc
+	 */
+	public function getHtml(): ParserOutput {
+		$message = $this->getDefaultSystemMessage();
+
+		// NOTE: This class should be used only for system messages,
+		//      so failing hard here is fine if we're not dealing with one.
+		$messageDom = DOMUtils::parseHTML( $message->parse() );
+		DOMUtils::appendToHead( $messageDom, 'meta', [
+			'http-equiv' => 'content-language',
+			'content' => LanguageCode::bcp47( $message->getLanguage()->getCode() ),
+		] );
+
+		$messageDocHtml = ContentUtils::toXML( $messageDom );
+
+		return new ParserOutput( $messageDocHtml );
+	}
+
+	/**
+	 * @inheritDoc
+	 */
+	public function getETag( string $suffix = '' ): ?string {
+		// XXX: We end up generating the HTML twice. Would be nice to avoid that.
+		// But messages are small, and not hit a lot...
+		$output = $this->getHtml();
+
+		return '"message/' . sha1( $output->getRawText() ) . '/' . $suffix . '"';
+	}
+
+	/**
+	 * @inheritDoc
+	 *
+	 * @note This is guaranteed to always return NULL since
+	 *   proper system messages (with no DB entry) have no
+	 *   revision, so they should have no last modified time.
+	 */
+	public function getLastModified(): ?string {
+		return null;
+	}
+
+	/**
+	 * @inheritDoc
+	 */
+	public function getParamSettings(): array {
+		return [];
+	}
+
+	/**
+	 * @inheritDoc
+	 */
+	public function setVariantConversionLanguage(
+		string $targetLanguageCode,
+		?string $sourceLanguageCode = null
+	): void {
+		// TODO: Set language in the response headers.
+	}
+
+	public function putHeaders(
+		ResponseInterface $response,
+		bool $forHtml = true
+	): void {
+		// TODO: Set language in the response headers.
+	}
+
+}
diff --git a/includes/Rest/Handler/HtmlOutputHelper.php b/includes/Rest/Handler/HtmlOutputHelper.php
new file mode 100644
index 00000000000..9d5dec441a0
--- /dev/null
+++ b/includes/Rest/Handler/HtmlOutputHelper.php
@@ -0,0 +1,98 @@
+<?php
+/**
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc.,
+ * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
+ * http://www.gnu.org/copyleft/gpl.html
+ *
+ * @file
+ */
+namespace MediaWiki\Rest\Handler;
+
+use MediaWiki\Rest\LocalizedHttpException;
+use MediaWiki\Rest\ResponseInterface;
+use ParserOutput;
+use Wikimedia\Parsoid\Core\ClientError;
+
+/**
+ * @since 1.40
+ * @unstable
+ */
+interface HtmlOutputHelper {
+
+	/**
+	 * Fetch the HTML for rendering of a given page. If the rendering is
+	 * available in parsoid parser cache, return that. Otherwise, perform
+	 * a parse and return the result while caching it in the parser cache.
+	 *
+	 * NOTE: Caching can be explicitly disabled or a force parse action
+	 *    can be issued. Stashing and rate limiting on stashing also applies
+	 *    here if specified.
+	 *
+	 * @return ParserOutput a tuple with html and content-type
+	 * @throws LocalizedHttpException
+	 * @throws ClientError
+	 */
+	public function getHtml(): ParserOutput;
+
+	/**
+	 * Returns an ETag uniquely identifying the HTML output.
+	 *
+	 * @see Handler::getETag()
+	 *
+	 * @param string $suffix A suffix to attach to the etag.
+	 *
+	 * @return string|null We return null when there is no etag.
+	 */
+	public function getETag( string $suffix = '' ): ?string;
+
+	/**
+	 * Returns the time at which the HTML was rendered.
+	 *
+	 * @see Handler::getLastModified()
+	 *
+	 * @return string|null
+	 */
+	public function getLastModified(): ?string;
+
+	/**
+	 * Gets the request parameters of this request.
+	 *
+	 * @see Handler::getParamSettings()
+	 *
+	 * @return array
+	 */
+	public function getParamSettings(): array;
+
+	/**
+	 * Set the language to be used for variant conversion.
+	 *
+	 * @param string $targetLanguageCode
+	 * @param null|string $sourceLanguageCode
+	 */
+	public function setVariantConversionLanguage(
+		string $targetLanguageCode,
+		?string $sourceLanguageCode = null
+	): void;
+
+	/**
+	 * Set the HTTP headers based on the response generated
+	 *
+	 * @param ResponseInterface $response
+	 * @param bool $forHtml Whether the response will be HTML (rather than JSON)
+	 *
+	 * @return void
+	 */
+	public function putHeaders( ResponseInterface $response, bool $forHtml = true ): void;
+
+}
diff --git a/includes/Rest/Handler/HtmlOutputRendererHelper.php b/includes/Rest/Handler/HtmlOutputRendererHelper.php
index b4a5c58dc8f..f4859cf5ae2 100644
--- a/includes/Rest/Handler/HtmlOutputRendererHelper.php
+++ b/includes/Rest/Handler/HtmlOutputRendererHelper.php
@@ -20,6 +20,7 @@
 namespace MediaWiki\Rest\Handler;
 
 use Content;
+use HttpError;
 use IBufferingStatsdDataFactory;
 use Language;
 use Liuggio\StatsdClient\Factory\StatsdDataFactoryInterface;
@@ -50,7 +51,6 @@ use User;
 use Wikimedia\Assert\Assert;
 use Wikimedia\Message\MessageValue;
 use Wikimedia\ParamValidator\ParamValidator;
-use Wikimedia\Parsoid\Core\ClientError;
 use Wikimedia\Parsoid\Core\PageBundle;
 use Wikimedia\Parsoid\Parsoid;
 use Wikimedia\Parsoid\Utils\ContentUtils;
@@ -63,7 +63,7 @@ use Wikimedia\Parsoid\Utils\DOMUtils;
  *
  * @unstable Pending consolidation of the Parsoid extension with core code.
  */
-class HtmlOutputRendererHelper {
+class HtmlOutputRendererHelper implements HtmlOutputHelper {
 	/**
 	 * @internal
 	 * @var string[]
@@ -271,6 +271,11 @@ class HtmlOutputRendererHelper {
 	 */
 	public function setRevision( $revisionOrId ): void {
 		Assert::parameterType( [ RevisionRecord::class, 'integer' ], $revisionOrId, '$revision' );
+
+		if ( is_int( $revisionOrId ) && $revisionOrId <= 0 ) {
+			throw new HttpError( 400, "Bad revision ID: $revisionOrId" );
+		}
+
 		$this->revisionOrId = $revisionOrId;
 
 		if ( $this->getRevisionId() === null ) {
@@ -335,11 +340,15 @@ class HtmlOutputRendererHelper {
 	}
 
 	/**
+	 * Initializes the helper with the given parameters like the page
+	 * we're dealing with, parameters gotten from the request inputs,
+	 * and the revision if any is available.
+	 *
 	 * @param PageIdentity $page
 	 * @param array $parameters
 	 * @param User $user
-	 * @param RevisionRecord|int|null $revision DEPRECATED, use setRevision()
-	 * @param Language|null $pageLanguage DEPRECATED, use setPageLanguage()
+	 * @param RevisionRecord|int|null $revision
+	 * @param Language|null $pageLanguage
 	 */
 	public function init(
 		PageIdentity $page,
@@ -350,10 +359,16 @@ class HtmlOutputRendererHelper {
 	) {
 		$this->page = $page;
 		$this->user = $user;
-		$this->revisionOrId = $revision;
-		$this->pageLanguage = $pageLanguage;
 		$this->stash = $parameters['stash'] ?? false;
 
+		if ( $revision !== null ) {
+			$this->setRevision( $revision );
+		}
+
+		if ( $pageLanguage !== null ) {
+			$this->setPageLanguage( $pageLanguage );
+		}
+
 		if ( $this->stash ) {
 			$this->setFlavor( 'stash' );
 		} else {
@@ -362,19 +377,18 @@ class HtmlOutputRendererHelper {
 	}
 
 	/**
-	 * Set the language to be used for variant conversion
-	 * @param string $targetLanguageCode
-	 * @param null|string $sourceLanguageCode
+	 * @inheritDoc
 	 */
-	public function setVariantConversionLanguage( string $targetLanguageCode, ?string $sourceLanguageCode = null ) {
+	public function setVariantConversionLanguage(
+		string $targetLanguageCode,
+		?string $sourceLanguageCode = null
+	): void {
 		$this->targetLanguageCode = $targetLanguageCode;
 		$this->sourceLanguageCode = $sourceLanguageCode;
 	}
 
 	/**
-	 * @return ParserOutput a tuple with html and content-type
-	 * @throws LocalizedHttpException
-	 * @throws ClientError
+	 * @inheritDoc
 	 */
 	public function getHtml(): ParserOutput {
 		if ( $this->processedParserOutput ) {
@@ -442,11 +456,7 @@ class HtmlOutputRendererHelper {
 	}
 
 	/**
-	 * Returns an ETag uniquely identifying the HTML output.
-	 *
-	 * @param string $suffix A suffix to attach to the etag.
-	 *
-	 * @return string|null
+	 * @inheritDoc
 	 */
 	public function getETag( string $suffix = '' ): ?string {
 		$parserOutput = $this->getParserOutput();
@@ -467,16 +477,14 @@ class HtmlOutputRendererHelper {
 	}
 
 	/**
-	 * Returns the time at which the HTML was rendered.
-	 *
-	 * @return string|null
+	 * @inheritDoc
 	 */
 	public function getLastModified(): ?string {
 		return $this->getParserOutput()->getCacheTime();
 	}
 
 	/**
-	 * @return array
+	 * @inheritDoc
 	 */
 	public function getParamSettings(): array {
 		return [
@@ -566,14 +574,9 @@ class HtmlOutputRendererHelper {
 	}
 
 	/**
-	 * Set the HTTP headers based on the response generated
-	 *
-	 * @param ResponseInterface $response
-	 * @param bool $forHtml Whether the response will be HTML (rather than JSON)
-	 *
-	 * @return void
+	 * @inheritDoc
 	 */
-	public function putHeaders( ResponseInterface $response, bool $forHtml = true ) {
+	public function putHeaders( ResponseInterface $response, bool $forHtml = true ): void {
 		if ( $forHtml ) {
 			// For HTML we want to set the Content-Language. For JSON, we probably don't.
 			$response->setHeader( 'Content-Language', $this->getHtmlOutputContentLanguage() );
diff --git a/includes/Rest/Handler/PageHTMLHandler.php b/includes/Rest/Handler/PageHTMLHandler.php
index 725b812eecb..45704200a68 100644
--- a/includes/Rest/Handler/PageHTMLHandler.php
+++ b/includes/Rest/Handler/PageHTMLHandler.php
@@ -2,7 +2,6 @@
 
 namespace MediaWiki\Rest\Handler;
 
-use LanguageCode;
 use LogicException;
 use MediaWiki\MediaWikiServices;
 use MediaWiki\Page\RedirectStore;
@@ -10,11 +9,8 @@ use MediaWiki\Rest\LocalizedHttpException;
 use MediaWiki\Rest\Response;
 use MediaWiki\Rest\SimpleHandler;
 use MediaWiki\Rest\StringStream;
-use ParserOutput;
 use TitleFormatter;
 use Wikimedia\Assert\Assert;
-use Wikimedia\Parsoid\Utils\ContentUtils;
-use Wikimedia\Parsoid\Utils\DOMUtils;
 
 /**
  * A handler that returns Parsoid HTML for the following routes:
@@ -26,7 +22,7 @@ use Wikimedia\Parsoid\Utils\DOMUtils;
 class PageHTMLHandler extends SimpleHandler {
 	use PageRedirectHandlerTrait;
 
-	/** @var HtmlOutputRendererHelper */
+	/** @var HtmlOutputHelper */
 	private $htmlHelper;
 
 	/** @var PageContentHelper */
@@ -38,6 +34,8 @@ class PageHTMLHandler extends SimpleHandler {
 	/** @var RedirectStore */
 	private $redirectStore;
 
+	private PageRestHelperFactory $helperFactory;
+
 	public function __construct(
 		TitleFormatter $titleFormatter,
 		RedirectStore $redirectStore,
@@ -46,6 +44,7 @@ class PageHTMLHandler extends SimpleHandler {
 		$this->titleFormatter = $titleFormatter;
 		$this->redirectStore = $redirectStore;
 		$this->contentHelper = $helperFactory->newPageContentHelper();
+		$this->helperFactory = $helperFactory;
 		$this->htmlHelper = $helperFactory->newHtmlOutputRendererHelper();
 	}
 
@@ -57,19 +56,23 @@ class PageHTMLHandler extends SimpleHandler {
 		$this->contentHelper->init( $user, $this->getValidatedParams() );
 
 		$page = $this->contentHelper->getPageIdentity();
+		$isSystemMessage = $this->contentHelper->useDefaultSystemMessage();
 
-		if ( $this->contentHelper->useDefaultSystemMessage() ) {
-			// We can't use the helper object with system messages.
-			// TODO: We should have an implementation of HtmlOutputRendererHelper
-			//       for system messages in the future.
-			// Currently NO OP.
-		} elseif ( $page ) {
-			$this->htmlHelper->init( $page, $this->getValidatedParams(), $user );
-
-			$request = $this->getRequest();
-			$acceptLanguage = $request->getHeaderLine( 'Accept-Language' ) ?: null;
-			if ( $acceptLanguage ) {
-				$this->htmlHelper->setVariantConversionLanguage( $acceptLanguage );
+		if ( $page ) {
+			if ( $isSystemMessage ) {
+				$this->htmlHelper = $this->helperFactory->newHtmlMessageOutputHelper();
+				$this->htmlHelper->init( $page );
+			} else {
+				$revision = $this->contentHelper->getTargetRevision();
+				// NOTE: We know that $this->htmlHelper is an instance of HtmlOutputRendererHelper
+				//       because we set it in the constructor.
+				$this->htmlHelper->init( $page, $this->getValidatedParams(), $user, $revision );
+
+				$request = $this->getRequest();
+				$acceptLanguage = $request->getHeaderLine( 'Accept-Language' ) ?: null;
+				if ( $acceptLanguage ) {
+					$this->htmlHelper->setVariantConversionLanguage( $acceptLanguage );
+				}
 			}
 		}
 	}
@@ -80,8 +83,7 @@ class PageHTMLHandler extends SimpleHandler {
 	 */
 	public function run(): Response {
 		$this->contentHelper->checkAccess();
-		$page = $this->contentHelper->getPage();
-		$isSystemMessage = $this->contentHelper->useDefaultSystemMessage();
+		$page = $this->contentHelper->getPageIdentity();
 		$params = $this->getRequest()->getQueryParams();
 
 		if ( array_key_exists( 'redirect', $params ) ) {
@@ -92,30 +94,22 @@ class PageHTMLHandler extends SimpleHandler {
 
 		// The call to $this->contentHelper->getPage() should not return null if
 		// $this->contentHelper->checkAccess() did not throw.
-		Assert::invariant(
-			$page !== null || $isSystemMessage,
-			'Page should be known or be a valid system message page'
+		Assert::invariant( $page !== null, 'Page should be known' );
+
+		$redirectResponse = $this->createRedirectResponseIfNeeded(
+			$page,
+			$followWikiRedirects,
+			$this->contentHelper->getTitleText(),
+			$this->titleFormatter,
+			$this->redirectStore
 		);
 
-		if ( $isSystemMessage ) {
-			$parserOutput = $this->getSystemMessageOutput();
-		} else {
-			'@phan-var \MediaWiki\Page\ExistingPageRecord $page';
-			$redirectResponse = $this->createRedirectResponseIfNeeded(
-				$page,
-				$followWikiRedirects,
-				$this->contentHelper->getTitleText(),
-				$this->titleFormatter,
-				$this->redirectStore
-			);
-
-			if ( $redirectResponse !== null ) {
-				return $redirectResponse;
-			}
-
-			$parserOutput = $this->htmlHelper->getHtml();
+		if ( $redirectResponse !== null ) {
+			return $redirectResponse;
 		}
 
+		$parserOutput = $this->htmlHelper->getHtml();
+
 		// Do not de-duplicate styles, Parsoid already does it in a slightly different way (T300325)
 		$parserOutputHtml = $parserOutput->getText( [ 'deduplicateStyles' => false ] );
 
@@ -130,17 +124,12 @@ class PageHTMLHandler extends SimpleHandler {
 				$body = $this->contentHelper->constructMetadata();
 				$body['html'] = $parserOutputHtml;
 
-				if ( $page ) {
-					// If param redirect=no is present, that means this page can be a redirect
-					// check for a redirectTargetUrl and send it to the body as `redirect_target`
-					'@phan-var \MediaWiki\Page\ExistingPageRecord $page';
-					$redirectTargetUrl = $this->getWikiRedirectTargetUrl(
-						$page, $this->redirectStore, $this->titleFormatter
-					);
-
-					if ( $redirectTargetUrl ) {
-						$body['redirect_target'] = $redirectTargetUrl;
-					}
+				$redirectTargetUrl = $this->getWikiRedirectTargetUrl(
+					$page, $this->redirectStore, $this->titleFormatter
+				);
+
+				if ( $redirectTargetUrl ) {
+					$body['redirect_target'] = $redirectTargetUrl;
 				}
 
 				$response = $this->getResponseFactory()->createJson( $body );
@@ -150,29 +139,12 @@ class PageHTMLHandler extends SimpleHandler {
 				throw new LogicException( "Unknown HTML type $outputMode" );
 		}
 
-		if ( !$isSystemMessage ) {
-			$setContentLanguageHeader = ( $outputMode === 'html' );
-			$this->htmlHelper->putHeaders( $response, $setContentLanguageHeader );
-		}
+		$setContentLanguageHeader = ( $outputMode === 'html' );
+		$this->htmlHelper->putHeaders( $response, $setContentLanguageHeader );
 
 		return $response;
 	}
 
-	private function getSystemMessageOutput(): ParserOutput {
-		$message = $this->contentHelper->getDefaultSystemMessage();
-
-		$messageDom = DOMUtils::parseHTML( $message->parse() );
-		DOMUtils::appendToHead( $messageDom, 'meta', [
-			'http-equiv' => 'content-language',
-			'content' => LanguageCode::bcp47( $message->getLanguage()->getCode() ),
-		] );
-
-		$messageDocHtml = ContentUtils::toXML( $messageDom );
-
-		// TODO: Set language in the response headers.
-		return new ParserOutput( $messageDocHtml );
-	}
-
 	/**
 	 * Returns an ETag representing a page's source. The ETag assumes a page's source has changed
 	 * if the latest revision of a page has been made private, un-readable for another reason,
@@ -184,13 +156,6 @@ class PageHTMLHandler extends SimpleHandler {
 			return null;
 		}
 
-		if ( $this->contentHelper->useDefaultSystemMessage() ) {
-			// XXX: We end up generating the HTML twice. Would be nice to avoid that.
-			// But messages are small, and not hit a lot...
-			$output = $this->getSystemMessageOutput();
-			return '"message/' . sha1( $output->getRawText() ) . '/' . $this->getOutputMode() . '"';
-		}
-
 		// Vary eTag based on output mode
 		return $this->htmlHelper->getETag( $this->getOutputMode() );
 	}
@@ -203,10 +168,6 @@ class PageHTMLHandler extends SimpleHandler {
 			return null;
 		}
 
-		if ( $this->contentHelper->useDefaultSystemMessage() ) {
-			return null;
-		}
-
 		return $this->htmlHelper->getLastModified();
 	}
 
diff --git a/includes/Rest/Handler/PageRestHelperFactory.php b/includes/Rest/Handler/PageRestHelperFactory.php
index 5967229fc71..d40df082252 100644
--- a/includes/Rest/Handler/PageRestHelperFactory.php
+++ b/includes/Rest/Handler/PageRestHelperFactory.php
@@ -99,6 +99,10 @@ class PageRestHelperFactory {
 		);
 	}
 
+	public function newHtmlMessageOutputHelper(): HtmlMessageOutputHelper {
+		return new HtmlMessageOutputHelper();
+	}
+
 	public function newHtmlInputTransformHelper( $envOptions = [] ): HtmlInputTransformHelper {
 		return new HtmlInputTransformHelper(
 			$this->stats,
diff --git a/includes/Rest/Handler/RevisionHTMLHandler.php b/includes/Rest/Handler/RevisionHTMLHandler.php
index 5c4b800fa16..bcd31589aa4 100644
--- a/includes/Rest/Handler/RevisionHTMLHandler.php
+++ b/includes/Rest/Handler/RevisionHTMLHandler.php
@@ -42,8 +42,7 @@ class RevisionHTMLHandler extends SimpleHandler {
 		$revision = $this->contentHelper->getTargetRevision();
 
 		if ( $page && $revision ) {
-			$this->htmlHelper->init( $page, $this->getValidatedParams(), $user );
-			$this->htmlHelper->setRevision( $revision );
+			$this->htmlHelper->init( $page, $this->getValidatedParams(), $user, $revision );
 
 			$request = $this->getRequest();
 			$acceptLanguage = $request->getHeaderLine( 'Accept-Language' ) ?: null;
diff --git a/tests/phpunit/integration/includes/Rest/Handler/HtmlMessageOutputHelperTest.php b/tests/phpunit/integration/includes/Rest/Handler/HtmlMessageOutputHelperTest.php
new file mode 100644
index 00000000000..c0392f1109f
--- /dev/null
+++ b/tests/phpunit/integration/includes/Rest/Handler/HtmlMessageOutputHelperTest.php
@@ -0,0 +1,71 @@
+<?php
+
+namespace MediaWiki\Tests\Rest\Helper;
+
+use MediaWiki\Rest\Handler\HtmlMessageOutputHelper;
+use MediaWikiIntegrationTestCase;
+
+/**
+ * @covers \MediaWiki\Rest\Handler\HtmlMessageOutputHelper
+ * @group Database
+ */
+class HtmlMessageOutputHelperTest extends MediaWikiIntegrationTestCase {
+	private function newHelper(): HtmlMessageOutputHelper {
+		return new HtmlMessageOutputHelper();
+	}
+
+	/**
+	 * @covers \MediaWiki\Rest\Handler\HtmlMessageOutputHelper::init
+	 * @covers \MediaWiki\Rest\Handler\HtmlMessageOutputHelper::getHtml
+	 */
+	public function testGetHtml() {
+		$page = $this->getNonexistingTestPage( 'MediaWiki:Logouttext' );
+
+		$helper = $this->newHelper();
+		$helper->init( $page );
+
+		$this->assertSame( 0, $page->getLatest() );
+
+		$htmlresult = $helper->getHtml()->getRawText();
+
+		$this->assertStringContainsString( 'You are now logged out', $htmlresult );
+		// Check that we have a full HTML document in English
+		$this->assertStringContainsString( '<html', $htmlresult );
+		$this->assertStringContainsString( 'content="en"', $htmlresult );
+	}
+
+	/**
+	 * @covers \MediaWiki\Rest\Handler\HtmlMessageOutputHelper::init
+	 * @covers \MediaWiki\Rest\Handler\HtmlMessageOutputHelper::getETag
+	 */
+	public function testGetETag() {
+		$page = $this->getNonexistingTestPage( 'MediaWiki:Logouttext' );
+
+		$helper = $this->newHelper();
+		$helper->init( $page );
+
+		$etag = $helper->getETag();
+
+		$this->assertStringContainsString( '"message/', $etag );
+	}
+
+	/**
+	 * @covers \MediaWiki\Rest\Handler\HtmlMessageOutputHelper::init
+	 * @covers \MediaWiki\Rest\Handler\HtmlMessageOutputHelper::getHtml
+	 */
+	public function testGetHtmlWithLanguageCode() {
+		$page = $this->getNonexistingTestPage( 'MediaWiki:Logouttext/de' );
+
+		$helper = $this->newHelper();
+		$helper->init( $page );
+
+		$this->assertSame( 0, $page->getLatest() );
+
+		$htmlresult = $helper->getHtml()->getRawText();
+
+		$this->assertStringContainsString( 'Du bist nun abgemeldet', $htmlresult );
+		// Check that we have a full HTML document in English
+		$this->assertStringContainsString( '<html', $htmlresult );
+		$this->assertStringContainsString( 'content="de"', $htmlresult );
+	}
+}
diff --git a/tests/phpunit/integration/includes/Rest/Handler/PageRestHelperFactoryTest.php b/tests/phpunit/integration/includes/Rest/Handler/PageRestHelperFactoryTest.php
new file mode 100644
index 00000000000..751c67f9bd9
--- /dev/null
+++ b/tests/phpunit/integration/includes/Rest/Handler/PageRestHelperFactoryTest.php
@@ -0,0 +1,33 @@
+<?php
+
+namespace MediaWiki\Tests\Rest\Handler;
+
+use MediaWiki\Rest\Handler\HtmlMessageOutputHelper;
+use MediaWiki\Rest\Handler\HtmlOutputHelper;
+use MediaWiki\Rest\Handler\HtmlOutputRendererHelper;
+use MediaWikiIntegrationTestCase;
+
+/**
+ * @covers \MediaWiki\Rest\Handler\PageRestHelperFactory
+ */
+class PageRestHelperFactoryTest extends MediaWikiIntegrationTestCase {
+
+	/**
+	 * @covers \MediaWiki\Rest\Handler\PageRestHelperFactory::newHtmlMessageOutputHelper
+	 * @covers \MediaWiki\Rest\Handler\PageRestHelperFactory::newHtmlOutputRendererHelper
+	 */
+	public function testNewHtmlOutputHelpers() {
+		$helperFactory = $this->getServiceContainer()->getPageRestHelperFactory();
+
+		$helper = $helperFactory->newHtmlMessageOutputHelper();
+
+		$this->assertInstanceOf( HtmlMessageOutputHelper::class, $helper );
+		$this->assertInstanceOf( HtmlOutputHelper::class, $helper );
+
+		$helper = $helperFactory->newHtmlOutputRendererHelper();
+
+		$this->assertInstanceOf( HtmlOutputRendererHelper::class, $helper );
+		$this->assertInstanceOf( HtmlOutputHelper::class, $helper );
+	}
+
+}
diff --git a/tests/phpunit/integration/includes/Rest/Handler/ParsoidHandlerTest.php b/tests/phpunit/integration/includes/Rest/Handler/ParsoidHandlerTest.php
index 59df8513d4e..c7e158e074d 100644
--- a/tests/phpunit/integration/includes/Rest/Handler/ParsoidHandlerTest.php
+++ b/tests/phpunit/integration/includes/Rest/Handler/ParsoidHandlerTest.php
@@ -1965,7 +1965,7 @@ class ParsoidHandlerTest extends MediaWikiIntegrationTestCase {
 	 * @dataProvider provideWt2html
 	 *
 	 * @param array $attribs
-	 * @param string $text
+	 * @param string|null $text
 	 * @param string[] $expectedData
 	 * @param string[] $unexpectedHtml
 	 * @param string[] $expectedHeaders
diff --git a/tests/phpunit/unit/includes/Rest/Handler/PageHandlerTestTrait.php b/tests/phpunit/unit/includes/Rest/Handler/PageHandlerTestTrait.php
index c43efd23df2..b67afeacc21 100644
--- a/tests/phpunit/unit/includes/Rest/Handler/PageHandlerTestTrait.php
+++ b/tests/phpunit/unit/includes/Rest/Handler/PageHandlerTestTrait.php
@@ -8,6 +8,7 @@ use MediaWiki\MainConfigNames;
 use MediaWiki\MainConfigSchema;
 use MediaWiki\Parser\ParserCacheFactory;
 use MediaWiki\Parser\Parsoid\ParsoidOutputAccess;
+use MediaWiki\Rest\Handler\HtmlMessageOutputHelper;
 use MediaWiki\Rest\Handler\HtmlOutputRendererHelper;
 use MediaWiki\Rest\Handler\LanguageLinksHandler;
 use MediaWiki\Rest\Handler\PageContentHelper;
@@ -83,7 +84,7 @@ trait PageHandlerTestTrait {
 
 		$helperFactory = $this->createNoOpMock(
 			PageRestHelperFactory::class,
-			[ 'newPageContentHelper', 'newHtmlOutputRendererHelper' ]
+			[ 'newPageContentHelper', 'newHtmlOutputRendererHelper', 'newHtmlMessageOutputHelper' ]
 		);
 
 		$helperFactory->method( 'newPageContentHelper' )
@@ -94,15 +95,20 @@ trait PageHandlerTestTrait {
 				$services->getPageStore()
 			) );
 
+		$parsoidOutputStash = $this->getParsoidOutputStash();
 		$helperFactory->method( 'newHtmlOutputRendererHelper' )
-			->willReturn( new HtmlOutputRendererHelper(
-				$this->getParsoidOutputStash(),
-				$services->getStatsdDataFactory(),
-				$parsoidOutputAccess,
-				$services->getHtmlTransformFactory(),
-				$services->getContentHandlerFactory(),
-				$services->getLanguageFactory()
-			) );
+			->willReturn(
+				new HtmlOutputRendererHelper(
+					$parsoidOutputStash,
+					$services->getStatsdDataFactory(),
+					$parsoidOutputAccess,
+					$services->getHtmlTransformFactory(),
+					$services->getContentHandlerFactory(),
+					$services->getLanguageFactory()
+				)
+			);
+		$helperFactory->method( 'newHtmlMessageOutputHelper' )
+			->willReturn( new HtmlMessageOutputHelper() );
 
 		return new PageHTMLHandler(
 			$services->getTitleFormatter(),