diff --git a/core/lib/Drupal/Core/Path/AliasStorageInterface.php b/core/lib/Drupal/Core/Path/AliasStorageInterface.php
index ae79300..5ac77a3 100644
--- a/core/lib/Drupal/Core/Path/AliasStorageInterface.php
+++ b/core/lib/Drupal/Core/Path/AliasStorageInterface.php
@@ -50,7 +50,7 @@ public function save($source, $alias, $langcode = LanguageInterface::LANGCODE_NO
    * @return array|false
    *   FALSE if no alias was found or an associative array containing the
    *   following keys:
-   *   - source (string): The internal system path with a starting lash.
+   *   - source (string): The internal system path with a starting slash.
    *   - alias (string): The URL alias with a starting slash.
    *   - pid (int): Unique path alias identifier.
    *   - langcode (string): The language code of the alias.
diff --git a/core/lib/Drupal/Core/PathProcessor/PathProcessorFront.php b/core/lib/Drupal/Core/PathProcessor/PathProcessorFront.php
index 3fdcbb7..bd1d8c1 100644
--- a/core/lib/Drupal/Core/PathProcessor/PathProcessorFront.php
+++ b/core/lib/Drupal/Core/PathProcessor/PathProcessorFront.php
@@ -50,7 +50,7 @@ public function processInbound($path, Request $request) {
    */
   public function processOutbound($path, &$options = array(), Request $request = NULL, CacheableMetadata $cacheable_metadata = NULL) {
     // The special path '<front>' links to the default front page.
-    if ($path == '/<front>') {
+    if ($path === '/<front>') {
       $path = '/';
     }
     return $path;
diff --git a/core/lib/Drupal/Core/Routing/RouteProvider.php b/core/lib/Drupal/Core/Routing/RouteProvider.php
index 9989274..278b486 100644
--- a/core/lib/Drupal/Core/Routing/RouteProvider.php
+++ b/core/lib/Drupal/Core/Routing/RouteProvider.php
@@ -142,7 +142,7 @@ public function getRouteCollectionForRequest(Request $request) {
     else {
       // Just trim on the right side.
       $path = $request->getPathInfo();
-      $path = $path == '/' ? $path : rtrim($request->getPathInfo(), '/');
+      $path = $path === '/' ? $path : rtrim($request->getPathInfo(), '/');
       $path = $this->pathProcessor->processInbound($path, $request);
       $this->currentPath->setPath($path, $request);
       // Incoming path processors may also set query parameters.
diff --git a/core/modules/config/src/Tests/ConfigSingleImportExportTest.php b/core/modules/config/src/Tests/ConfigSingleImportExportTest.php
index 8c8594c..688d867 100644
--- a/core/modules/config/src/Tests/ConfigSingleImportExportTest.php
+++ b/core/modules/config/src/Tests/ConfigSingleImportExportTest.php
@@ -27,7 +27,7 @@ class ConfigSingleImportExportTest extends WebTestBase {
   /**
    * Tests importing a single configuration file.
    */
-  public function tesImport() {
+  public function testImport() {
     $storage = \Drupal::entityManager()->getStorage('config_test');
     $uuid = \Drupal::service('uuid');
 
@@ -141,7 +141,7 @@ public function testImportSimpleConfiguration() {
   /**
    * Tests exporting a single configuration file.
    */
-  public function tesExport() {
+  public function testExport() {
     $this->drupalLogin($this->drupalCreateUser(array('export configuration')));
 
     $this->drupalGet('admin/config/development/configuration/single/export/system.simple');
diff --git a/core/modules/path/src/Tests/PathAliasTest.php b/core/modules/path/src/Tests/PathAliasTest.php
index 3708aa6..e4d07e2 100644
--- a/core/modules/path/src/Tests/PathAliasTest.php
+++ b/core/modules/path/src/Tests/PathAliasTest.php
@@ -176,6 +176,18 @@ function testAdminAlias() {
     $this->drupalPostForm('admin/config/search/path/edit/' . $pid, $edit, t('Save'));
     $this->assertRaw(t('The alias %alias is already in use in this language.', array('%alias' => $edit['alias'])));
 
+    // Create an alias without a starting slash.
+    $node5 = $this->drupalCreateNode();
+
+    $edit = array();
+    $edit['source'] = 'node/' . $node5->id();
+    $node5_alias = $this->randomMachineName(8);
+    $edit['alias'] = $node5_alias . '/';
+    $this->drupalPostForm('admin/config/search/path/add', $edit, t('Save'));
+
+    $this->assertUrl('admin/config/search/path/add');
+    $this->assertText('The source path has to start with a slash.');
+    $this->assertText('The alias path has to start with a slash.');
   }
 
   /**
diff --git a/core/modules/system/src/Form/SiteInformationForm.php b/core/modules/system/src/Form/SiteInformationForm.php
index af1c83d..1b1e3a6 100644
--- a/core/modules/system/src/Form/SiteInformationForm.php
+++ b/core/modules/system/src/Form/SiteInformationForm.php
@@ -173,7 +173,7 @@ public function validateForm(array &$form, FormStateInterface $form_state) {
     }
     // Validate front page path.
     if (($value = $form_state->getValue('site_frontpage')) && $value[0] !== '/') {
-      $form_state->setErrorByName('site_frontpage', $this->t("The path '%path' has to start with a /.", ['%path' => $form_state->getValue('site_frontpage')]));
+      $form_state->setErrorByName('site_frontpage', $this->t("The path '%path' has to start with a slash.", ['%path' => $form_state->getValue('site_frontpage')]));
 
     }
     if (!$this->pathValidator->isValid($form_state->getValue('site_frontpage'))) {
@@ -187,10 +187,10 @@ public function validateForm(array &$form, FormStateInterface $form_state) {
       $form_state->setValueForElement($form['error_page']['site_404'], $this->aliasManager->getPathByAlias($form_state->getValue('site_404')));
     }
     if (($value = $form_state->getValue('site_403')) && $value[0] !== '/') {
-      $form_state->setErrorByName('site_403', $this->t("The path '%path' has to start with a /.", ['%path' => $form_state->getValue('site_403')]));
+      $form_state->setErrorByName('site_403', $this->t("The path '%path' has to start with a slash.", ['%path' => $form_state->getValue('site_403')]));
     }
     if (($value = $form_state->getValue('site_404')) && $value[0] !== '/') {
-      $form_state->setErrorByName('site_404', $this->t("The path '%path' has to start with a /.", ['%path' => $form_state->getValue('site_404')]));
+      $form_state->setErrorByName('site_404', $this->t("The path '%path' has to start with a slash.", ['%path' => $form_state->getValue('site_404')]));
     }
     // Validate 403 error path.
     if (!$form_state->isValueEmpty('site_403') && !$this->pathValidator->isValid($form_state->getValue('site_403'))) {
diff --git a/core/modules/system/src/Tests/System/AccessDeniedTest.php b/core/modules/system/src/Tests/System/AccessDeniedTest.php
index 1553421..0f37320 100644
--- a/core/modules/system/src/Tests/System/AccessDeniedTest.php
+++ b/core/modules/system/src/Tests/System/AccessDeniedTest.php
@@ -7,6 +7,7 @@
 
 namespace Drupal\system\Tests\System;
 
+use Drupal\Component\Utility\SafeMarkup;
 use Drupal\simpletest\WebTestBase;
 use Drupal\user\RoleInterface;
 
@@ -41,8 +42,16 @@ function testAccessDenied() {
     $this->assertText(t('Access denied'), 'Found the default 403 page');
     $this->assertResponse(403);
 
-    // Use a custom 403 page.
     $this->drupalLogin($this->adminUser);
+
+    // Set a custom 404 page without a starting slash.
+    $edit = [
+      'site_403' => 'user/' . $this->adminUser->id(),
+    ];
+    $this->drupalPostForm('admin/config/system/site-information', $edit, t('Save configuration'));
+    $this->assertRaw(SafeMarkup::format("The path '%path' has to start with a slash.", ['%path' =>  $edit['site_403']]));
+
+    // Use a custom 403 page.
     $edit = [
       'site_403' => '/user/' . $this->adminUser->id(),
     ];
diff --git a/core/modules/system/src/Tests/System/FrontPageTest.php b/core/modules/system/src/Tests/System/FrontPageTest.php
index 34a3aeb..88235b3 100644
--- a/core/modules/system/src/Tests/System/FrontPageTest.php
+++ b/core/modules/system/src/Tests/System/FrontPageTest.php
@@ -71,6 +71,11 @@ public function testDrupalFrontPage() {
     $this->drupalPostForm('admin/config/system/site-information', $edit, t('Save configuration'));
     $this->assertText(t("The path '@path' is either invalid or you do not have access to it.", array('@path' => $edit['site_frontpage'])));
 
+    // Change the front page to a path without a starting slash.
+    $edit = ['site_frontpage' => $this->nodePath];
+    $this->drupalPostForm('admin/config/system/site-information', $edit, t('Save configuration'));
+    $this->assertRaw(SafeMarkup::format("The path '%path' has to start with a slash.", ['%path' =>  $edit['site_frontpage']]));
+
     // Change the front page to a valid path.
     $edit['site_frontpage'] = '/' . $this->nodePath;
     $this->drupalPostForm('admin/config/system/site-information', $edit, t('Save configuration'));
diff --git a/core/modules/system/src/Tests/System/PageNotFoundTest.php b/core/modules/system/src/Tests/System/PageNotFoundTest.php
index 7ebff3e..b7c0609 100644
--- a/core/modules/system/src/Tests/System/PageNotFoundTest.php
+++ b/core/modules/system/src/Tests/System/PageNotFoundTest.php
@@ -7,6 +7,7 @@
 
 namespace Drupal\system\Tests\System;
 
+use Drupal\Component\Utility\SafeMarkup;
 use Drupal\simpletest\WebTestBase;
 use Drupal\user\RoleInterface;
 
@@ -33,6 +34,13 @@ function testPageNotFound() {
     $this->drupalGet($this->randomMachineName(10));
     $this->assertText(t('Page not found'), 'Found the default 404 page');
 
+    // Set a custom 404 page without a starting slash.
+    $edit = [
+      'site_404' => 'user/' . $this->adminUser->id(),
+    ];
+    $this->drupalPostForm('admin/config/system/site-information', $edit, t('Save configuration'));
+    $this->assertRaw(SafeMarkup::format("The path '%path' has to start with a slash.", ['%path' =>  $edit['site_404']]));
+
     // Use a custom 404 page.
     $edit = array(
       'site_404' => '/user/' . $this->adminUser->id(),