React + Material-UI - Warning: Prop className did not match

The problem is the SSR rendering in Next.js, which produces the style fragment before the page is rendered.

Using Material UI and Next.js (as the author is using), adding a file called _document.js solved the problem.

Adjusted _document.js (as suggested here):

import React from 'react';
import Document, { Html, Head, Main, NextScript } from 'next/document';
import { ServerStyleSheets } from '@material-ui/styles'; // works with @material-ui/core/styles, if you prefer to use it.
import theme from '../src/theme'; // Adjust here as well

export default class MyDocument extends Document {
  render() {
    return (
      <Html lang="en">
          {/* Not exactly required, but this is the PWA primary color */}
          <meta name="theme-color" content={theme.palette.primary.main} />
          <Main />
          <NextScript />

// `getInitialProps` belongs to `_document` (instead of `_app`),
// it's compatible with server-side generation (SSG).
MyDocument.getInitialProps = async (ctx) => {
  // Resolution order
  // On the server:
  // 1. app.getInitialProps
  // 2. page.getInitialProps
  // 3. document.getInitialProps
  // 4. app.render
  // 5. page.render
  // 6. document.render
  // On the server with error:
  // 1. document.getInitialProps
  // 2. app.render
  // 3. page.render
  // 4. document.render
  // On the client
  // 1. app.getInitialProps
  // 2. page.getInitialProps
  // 3. app.render
  // 4. page.render

  // Render app and page and get the context of the page with collected side effects.
  const sheets = new ServerStyleSheets();
  const originalRenderPage = ctx.renderPage;

  ctx.renderPage = () =>
      enhanceApp: (App) => (props) => sheets.collect(<App {...props} />),

  const initialProps = await Document.getInitialProps(ctx);

  return {
    // Styles fragment is rendered after the app and page rendering finish.
    styles: [...React.Children.toArray(initialProps.styles), sheets.getStyleElement()],

The issue is the server side generates the class names but style sheets are not automatically included in the HTML. You need to explicitly extract the CSS and append it to the UI for the server side rendered components. The whole process is explained here:

This problem is related to MUI using dynamic class name which contain an ID. The IDs from the server side rendered CSS are not the same as the client side CSS, hence the mismatch error. A good start is to read the MUI SSR documentation

If you have this problem with nextjs (as I did) follow the example provided by the MUI team, which can be found here: material-ui/examples/nextjs

The most important part is in "examples/nextjs/pages/_app.js":

componentDidMount() {
    // Remove the server-side injected CSS.
    const jssStyles = document.querySelector('#jss-server-side');
    if (jssStyles) {

the related ticket can be found here: mui-org/material-ui/issues/15073

what it does, is remove the server side rendered stylesheet and replace it by a new client side rendered one