diff --git a/packages/docusaurus-plugin-content-blog/src/pluginOptionSchema.ts b/packages/docusaurus-plugin-content-blog/src/pluginOptionSchema.ts
index ea11aaa22d..6fd4c3581c 100644
--- a/packages/docusaurus-plugin-content-blog/src/pluginOptionSchema.ts
+++ b/packages/docusaurus-plugin-content-blog/src/pluginOptionSchema.ts
@@ -28,7 +28,7 @@ export const DEFAULT_OPTIONS = {
 
 export const PluginOptionSchema = Joi.object({
   path: Joi.string().default(DEFAULT_OPTIONS.path),
-  routeBasePath: Joi.string().default(DEFAULT_OPTIONS.routeBasePath),
+  routeBasePath: Joi.string().allow('').default(DEFAULT_OPTIONS.routeBasePath),
   include: Joi.array().items(Joi.string()).default(DEFAULT_OPTIONS.include),
   postsPerPage: Joi.number()
     .integer()
@@ -45,16 +45,19 @@ export const PluginOptionSchema = Joi.object({
   showReadingTime: Joi.bool().default(DEFAULT_OPTIONS.showReadingTime),
   remarkPlugins: Joi.array()
     .items(
-      Joi.alternatives().try(
-        Joi.function(),
-        Joi.array()
-          .items(Joi.function().required(), Joi.object().required())
-          .length(2),
-      ),
+      Joi.array()
+        .items(Joi.function().required(), Joi.object().required())
+        .length(2),
+      Joi.function(),
     )
     .default(DEFAULT_OPTIONS.remarkPlugins),
   rehypePlugins: Joi.array()
-    .items(Joi.string())
+    .items(
+      Joi.array()
+        .items(Joi.function().required(), Joi.object().required())
+        .length(2),
+      Joi.function(),
+    )
     .default(DEFAULT_OPTIONS.rehypePlugins),
   editUrl: Joi.string().uri(),
   truncateMarker: Joi.object().default(DEFAULT_OPTIONS.truncateMarker),
diff --git a/packages/docusaurus-plugin-content-docs/src/pluginOptionSchema.ts b/packages/docusaurus-plugin-content-docs/src/pluginOptionSchema.ts
index 1a95d4f298..93739252b3 100644
--- a/packages/docusaurus-plugin-content-docs/src/pluginOptionSchema.ts
+++ b/packages/docusaurus-plugin-content-docs/src/pluginOptionSchema.ts
@@ -29,7 +29,7 @@ export const DEFAULT_OPTIONS: PluginOptions = {
 export const PluginOptionSchema = Joi.object({
   path: Joi.string().default(DEFAULT_OPTIONS.path),
   editUrl: Joi.string().uri(),
-  routeBasePath: Joi.string().default(DEFAULT_OPTIONS.routeBasePath),
+  routeBasePath: Joi.string().allow('').default(DEFAULT_OPTIONS.routeBasePath),
   homePageId: Joi.string().default(DEFAULT_OPTIONS.homePageId),
   include: Joi.array().items(Joi.string()).default(DEFAULT_OPTIONS.include),
   sidebarPath: Joi.string().default(DEFAULT_OPTIONS.sidebarPath),
@@ -37,12 +37,19 @@ export const PluginOptionSchema = Joi.object({
   docItemComponent: Joi.string().default(DEFAULT_OPTIONS.docItemComponent),
   remarkPlugins: Joi.array()
     .items(
-      Joi.array().items(Joi.function(), Joi.object()).length(2),
+      Joi.array()
+        .items(Joi.function().required(), Joi.object().required())
+        .length(2),
       Joi.function(),
     )
     .default(DEFAULT_OPTIONS.remarkPlugins),
   rehypePlugins: Joi.array()
-    .items(Joi.string())
+    .items(
+      Joi.array()
+        .items(Joi.function().required(), Joi.object().required())
+        .length(2),
+      Joi.function(),
+    )
     .default(DEFAULT_OPTIONS.rehypePlugins),
   showLastUpdateTime: Joi.bool().default(DEFAULT_OPTIONS.showLastUpdateTime),
   showLastUpdateAuthor: Joi.bool().default(
diff --git a/packages/docusaurus-theme-classic/src/themeConfigSchema.js b/packages/docusaurus-theme-classic/src/themeConfigSchema.js
index 38af74c70e..c6e13b868f 100644
--- a/packages/docusaurus-theme-classic/src/themeConfigSchema.js
+++ b/packages/docusaurus-theme-classic/src/themeConfigSchema.js
@@ -135,7 +135,7 @@ const ThemeConfigSchema = Joi.object({
         'themeConfig.navbar.links has been renamed as themeConfig.navbar.items',
     }),
     items: Joi.array().items(NavbarItemSchema),
-    title: Joi.string().required(),
+    title: Joi.string().allow('', null),
     logo: Joi.object({
       alt: Joi.string(),
       src: Joi.string().required(),
diff --git a/packages/docusaurus/src/server/configValidation.ts b/packages/docusaurus/src/server/configValidation.ts
index 81db67f910..8cd1874648 100644
--- a/packages/docusaurus/src/server/configValidation.ts
+++ b/packages/docusaurus/src/server/configValidation.ts
@@ -6,8 +6,8 @@
  */
 
 import {DocusaurusConfig} from '@docusaurus/types';
-import Joi from '@hapi/joi';
 import {CONFIG_FILE_NAME} from '../constants';
+import Joi from '@hapi/joi';
 
 export const DEFAULT_CONFIG: Pick<
   DocusaurusConfig,
@@ -66,14 +66,14 @@ const ConfigSchema = Joi.object({
       src: Joi.string().required(),
       async: Joi.bool(),
       defer: Joi.bool(),
-    }).oxor('async', 'defer'),
+    }),
   ),
   stylesheets: Joi.array().items(
     Joi.string(),
     Joi.object({
-      href: Joi.string().uri().required(),
+      href: Joi.string().required(),
       type: Joi.string().required(),
-    }),
+    }).unknown(),
   ),
   tagline: Joi.string(),
 });