[material-ui][CssBaseline] Margin not being set to 0 on headers in Next.js

[vim-diesel]

Steps to reproduce

Link to live example: https://space-traders-orpin.vercel.app/

View code here:
vim-diesel/mui-issue-github-example3

Steps:

1. Start a NextJS project using latest version, 14, and install MUI
2. import { AppRouterCacheProvider } from '@mui/material-nextjs/v14-appRouter'; and wrap it around the {children} in layout
3. Create a Provider client component and import { ThemeProvider, createTheme } from '@mui/material/styles'; and use it in the layout
4. Import the CssBaseline into the Layout component so it is rendered alongside all {children}.
5. Create a header and it will have a margin.

Current behavior

There is a margin applied to all headers.

Expected behavior

A proper CSS Reset should remove this.

Context

Open up the live example and remove the rule that applies a margin of 0 to all element and the margin gets appears. This should already be applied by a css reset.

Your environment

npx @mui/envinfo

System:

OS: Linux 5.15 Ubuntu 22.04.3 LTS 22.04.3 LTS (Jammy Jellyfish)

Binaries:

Node: 20.9.0 - ~/.nvm/versions/node/v20.9.0/bin/node
Yarn: 1.22.19 - ~/.nvm/versions/node/v20.9.0/bin/yarn
npm: 10.2.3 - ~/.nvm/versions/node/v20.9.0/bin/npm

Browsers:

Chrome: Not Found

npmPackages:

@emotion/react: ^11.11.3 => 11.11.3
@emotion/styled: ^11.11.0 => 11.11.0
@mui/base:  5.0.0-beta.29
@mui/core-downloads-tracker:  5.15.2
@mui/material: ^5.15.2 => 5.15.2
@mui/material-nextjs: ^5.15.0 => 5.15.0
@mui/private-theming:  5.15.2
@mui/styled-engine:  5.15.2
@mui/system:  5.15.2
@mui/types:  7.2.11
@mui/utils:  5.15.2
@types/react:  18.2.46
react: ^18 => 18.2.0
react-dom: ^18 => 18.2.0
typescript:  5.3.3

  Don't forget to mention which browser you used. ===> Edge
  Output from `npx @mui/envinfo` goes here.

Search keywords: cssbaseline, css, margin, padding

[siriwatknp]

I could not see any margin applied. Feel free to reopen the issue with a screenshot or video that shows the issue.