482fccb922
### Update
We removed the `<head>` element checking for root layout in #41621. Since we also need `<head>` for preload in the future, and also css-in-js will require that. We're adding back the `head` element checking to make sure user always provide valid root layout including it.
### Issue
An issue was reported [here](https://github.com/mui/material-ui/issues/34905#issuecomment-1331945868) that the Emotion/MUI site was suffering from FOUC.
After an inspection, I noticed that the SSRed HTML didn't contain the inserted styles at all - despite them being inserted through `useServerInsertedHTML`. I managed to debug it down and discovered that their layout was missing `<head></head>` and thus the stream transformer skipped the insertion altogether cause of this check:
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| .. | ||
| .stats-app | ||
| __mocks__ | ||
| development | ||
| e2e | ||
| integration | ||
| lib | ||
| production | ||
| unit | ||
| .gitignore | ||
| jest-setup-after-env.ts | ||
| jest.d.ts | ||
| readme.md | ||
| test-file.txt | ||