diff --git a/packages/docusaurus-plugin-content-docs/src/sidebars/__tests__/__snapshots__/generator.test.ts.snap b/packages/docusaurus-plugin-content-docs/src/sidebars/__tests__/__snapshots__/generator.test.ts.snap
index e042c366581d..29dd570e2366 100644
--- a/packages/docusaurus-plugin-content-docs/src/sidebars/__tests__/__snapshots__/generator.test.ts.snap
+++ b/packages/docusaurus-plugin-content-docs/src/sidebars/__tests__/__snapshots__/generator.test.ts.snap
@@ -86,6 +86,9 @@ exports[`DefaultSidebarItemsGenerator generates simple flat sidebar 1`] = `
     "type": "doc",
   },
   {
+    "customProps": {
+      "custom": "prop",
+    },
     "id": "doc1",
     "label": "doc1 sidebar label",
     "type": "doc",
diff --git a/packages/docusaurus-plugin-content-docs/src/sidebars/__tests__/generator.test.ts b/packages/docusaurus-plugin-content-docs/src/sidebars/__tests__/generator.test.ts
index 43e40938df0e..f8fb292874c2 100644
--- a/packages/docusaurus-plugin-content-docs/src/sidebars/__tests__/generator.test.ts
+++ b/packages/docusaurus-plugin-content-docs/src/sidebars/__tests__/generator.test.ts
@@ -64,6 +64,7 @@ describe('DefaultSidebarItemsGenerator', () => {
           sidebarPosition: 2,
           frontMatter: {
             sidebar_label: 'doc1 sidebar label',
+            sidebar_custom_props: {custom: 'prop'},
           },
           title: '',
           unversionedId: 'doc1',
diff --git a/packages/docusaurus-plugin-content-docs/src/sidebars/generator.ts b/packages/docusaurus-plugin-content-docs/src/sidebars/generator.ts
index 7dc6ece7bedc..5a6b325449b1 100644
--- a/packages/docusaurus-plugin-content-docs/src/sidebars/generator.ts
+++ b/packages/docusaurus-plugin-content-docs/src/sidebars/generator.ts
@@ -138,7 +138,11 @@ Available doc IDs:
     ): WithPosition<SidebarItemDoc> {
       const {
         sidebarPosition: position,
-        frontMatter: {sidebar_label: label, sidebar_class_name: className},
+        frontMatter: {
+          sidebar_label: label,
+          sidebar_class_name: className,
+          sidebar_custom_props: customProps,
+        },
       } = getDoc(id);
       return {
         type: 'doc',
@@ -149,6 +153,7 @@ Available doc IDs:
         // sidebar
         ...(label !== undefined && {label}),
         ...(className !== undefined && {className}),
+        ...(customProps !== undefined && {customProps}),
       };
     }
     function createCategoryItem(
diff --git a/packages/docusaurus-theme-classic/src/theme/BlogPostItem/Container/index.tsx b/packages/docusaurus-theme-classic/src/theme/BlogPostItem/Container/index.tsx
index ab9d2a986933..0cfb6b336fe5 100644
--- a/packages/docusaurus-theme-classic/src/theme/BlogPostItem/Container/index.tsx
+++ b/packages/docusaurus-theme-classic/src/theme/BlogPostItem/Container/index.tsx
@@ -14,17 +14,26 @@ export default function BlogPostItemContainer({
   children,
   className,
 }: Props): JSX.Element {
-  const {frontMatter, assets} = useBlogPost();
+  const {
+    frontMatter,
+    assets,
+    metadata: {description},
+  } = useBlogPost();
   const {withBaseUrl} = useBaseUrlUtils();
   const image = assets.image ?? frontMatter.image;
+  const keywords = frontMatter.keywords ?? [];
   return (
     <article
       className={className}
       itemProp="blogPost"
       itemScope
       itemType="http://schema.org/BlogPosting">
+      {description && <meta itemProp="description" content={description} />}
       {image && (
-        <meta itemProp="image" content={withBaseUrl(image, {absolute: true})} />
+        <link itemProp="image" href={withBaseUrl(image, {absolute: true})} />
+      )}
+      {keywords.length > 0 && (
+        <meta itemProp="keywords" content={keywords.join(',')} />
       )}
       {children}
     </article>
diff --git a/packages/docusaurus-theme-classic/src/theme/BlogPostItem/Header/Author/index.tsx b/packages/docusaurus-theme-classic/src/theme/BlogPostItem/Header/Author/index.tsx
index 92ace43f3220..5f2eb1d7be38 100644
--- a/packages/docusaurus-theme-classic/src/theme/BlogPostItem/Header/Author/index.tsx
+++ b/packages/docusaurus-theme-classic/src/theme/BlogPostItem/Header/Author/index.tsx
@@ -28,7 +28,12 @@ export default function BlogPostItemHeaderAuthor({
     <div className={clsx('avatar margin-bottom--sm', className)}>
       {imageURL && (
         <MaybeLink href={link} className="avatar__photo-link">
-          <img className="avatar__photo" src={imageURL} alt={name} />
+          <img
+            className="avatar__photo"
+            src={imageURL}
+            alt={name}
+            itemProp="image"
+          />
         </MaybeLink>
       )}
 
diff --git a/packages/docusaurus-theme-classic/src/theme/DocPage/Layout/styles.module.css b/packages/docusaurus-theme-classic/src/theme/DocPage/Layout/styles.module.css
index 3cc2c95bdd7f..e5435c64edd7 100644
--- a/packages/docusaurus-theme-classic/src/theme/DocPage/Layout/styles.module.css
+++ b/packages/docusaurus-theme-classic/src/theme/DocPage/Layout/styles.module.css
@@ -15,3 +15,11 @@
   display: flex;
   flex: 1 0 auto;
 }
+
+/*
+JS disabled??? Show light version by default => better than showing nothing
+TODO bad, but we currently always show light mode when there's no data-theme
+ */
+html:not([data-theme]) .themedComponent--light {
+  display: initial;
+}
diff --git a/packages/docusaurus-theme-classic/src/theme/MDXComponents/Ul/index.tsx b/packages/docusaurus-theme-classic/src/theme/MDXComponents/Ul/index.tsx
index d426298f6a8d..4c76343602b9 100644
--- a/packages/docusaurus-theme-classic/src/theme/MDXComponents/Ul/index.tsx
+++ b/packages/docusaurus-theme-classic/src/theme/MDXComponents/Ul/index.tsx
@@ -11,7 +11,11 @@ import type {Props} from '@theme/MDXComponents/Ul';
 
 import styles from './styles.module.css';
 
-function transformUlClassName(className?: string): string {
+function transformUlClassName(className?: string): string | undefined {
+  // Fix https://github.com/facebook/docusaurus/issues/9098
+  if (typeof className === 'undefined') {
+    return undefined;
+  }
   return clsx(
     className,
     // This class is set globally by GitHub/MDX. We keep the global class, and
diff --git a/packages/docusaurus-theme-classic/src/theme/SiteMetadata/index.tsx b/packages/docusaurus-theme-classic/src/theme/SiteMetadata/index.tsx
index 9a30c775fff2..4a275e819c17 100644
--- a/packages/docusaurus-theme-classic/src/theme/SiteMetadata/index.tsx
+++ b/packages/docusaurus-theme-classic/src/theme/SiteMetadata/index.tsx
@@ -16,6 +16,7 @@ import {
   keyboardFocusedClassName,
 } from '@docusaurus/theme-common/internal';
 import {useLocation} from '@docusaurus/router';
+import {applyTrailingSlash} from '@docusaurus/utils-common';
 import SearchMetadata from '@theme/SearchMetadata';
 
 // TODO move to SiteMetadataDefaults or theme-common ?
@@ -58,10 +59,19 @@ function AlternateLangHeaders(): JSX.Element {
 // Default canonical url inferred from current page location pathname
 function useDefaultCanonicalUrl() {
   const {
-    siteConfig: {url: siteUrl},
+    siteConfig: {url: siteUrl, baseUrl, trailingSlash},
   } = useDocusaurusContext();
+
+  // TODO using useLocation().pathname is not a super idea
+  // See https://github.com/facebook/docusaurus/issues/9170
   const {pathname} = useLocation();
-  return siteUrl + useBaseUrl(pathname);
+
+  const canonicalPathname = applyTrailingSlash(useBaseUrl(pathname), {
+    trailingSlash,
+    baseUrl,
+  });
+
+  return siteUrl + canonicalPathname;
 }
 
 // TODO move to SiteMetadataDefaults or theme-common ?
diff --git a/packages/docusaurus-theme-common/src/utils/useAlternatePageUtils.ts b/packages/docusaurus-theme-common/src/utils/useAlternatePageUtils.ts
index ae0e55dcddff..073aa8956f3b 100644
--- a/packages/docusaurus-theme-common/src/utils/useAlternatePageUtils.ts
+++ b/packages/docusaurus-theme-common/src/utils/useAlternatePageUtils.ts
@@ -7,6 +7,7 @@
 
 import useDocusaurusContext from '@docusaurus/useDocusaurusContext';
 import {useLocation} from '@docusaurus/router';
+import {applyTrailingSlash} from '@docusaurus/utils-common';
 
 /**
  * Permits to obtain the url of the current page in another locale, useful to
@@ -35,17 +36,25 @@ export function useAlternatePageUtils(): {
   }) => string;
 } {
   const {
-    siteConfig: {baseUrl, url},
+    siteConfig: {baseUrl, url, trailingSlash},
     i18n: {defaultLocale, currentLocale},
   } = useDocusaurusContext();
+
+  // TODO using useLocation().pathname is not a super idea
+  // See https://github.com/facebook/docusaurus/issues/9170
   const {pathname} = useLocation();
 
+  const canonicalPathname = applyTrailingSlash(pathname, {
+    trailingSlash,
+    baseUrl,
+  });
+
   const baseUrlUnlocalized =
     currentLocale === defaultLocale
       ? baseUrl
       : baseUrl.replace(`/${currentLocale}/`, '/');
 
-  const pathnameSuffix = pathname.replace(baseUrl, '');
+  const pathnameSuffix = canonicalPathname.replace(baseUrl, '');
 
   function getLocalizedBaseUrl(locale: string) {
     return locale === defaultLocale
diff --git a/packages/docusaurus/src/client/App.tsx b/packages/docusaurus/src/client/App.tsx
index 8554f5b91aec..3a93ec6524a8 100644
--- a/packages/docusaurus/src/client/App.tsx
+++ b/packages/docusaurus/src/client/App.tsx
@@ -23,6 +23,7 @@ import SiteMetadataDefaults from './SiteMetadataDefaults';
 // TODO, quick fix for CSS insertion order
 // eslint-disable-next-line import/order
 import ErrorBoundary from '@docusaurus/ErrorBoundary';
+import HasHydratedDataAttribute from './hasHydratedDataAttribute';
 
 export default function App(): JSX.Element {
   const routeElement = renderRoutes(routes);
@@ -39,6 +40,7 @@ export default function App(): JSX.Element {
               {routeElement}
             </PendingNavigation>
           </Root>
+          <HasHydratedDataAttribute />
         </BrowserContextProvider>
       </DocusaurusContextProvider>
     </ErrorBoundary>
diff --git a/packages/docusaurus/src/client/hasHydratedDataAttribute.tsx b/packages/docusaurus/src/client/hasHydratedDataAttribute.tsx
new file mode 100644
index 000000000000..1855f059775e
--- /dev/null
+++ b/packages/docusaurus/src/client/hasHydratedDataAttribute.tsx
@@ -0,0 +1,21 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+import React from 'react';
+import Head from '@docusaurus/Head';
+import useIsBrowser from '@docusaurus/useIsBrowser';
+
+// See https://github.com/facebook/docusaurus/pull/9256
+// Docusaurus adds a <html data-has-hydrated="true"> after hydration
+export default function HasHydratedDataAttribute(): JSX.Element {
+  const isBrowser = useIsBrowser();
+  return (
+    <Head>
+      <html data-has-hydrated={isBrowser} />
+    </Head>
+  );
+}