Try it in a private window (with extensions disabled) and see if it happens

Do note also that it is just a warning and doesn't affect production at all

Thank you, can you please tell me how to you recognize the solution instantly.

The only difference found between the server and client html is that a class was added on the client. It is possible only if you have some weird logic that adds a class based on some web API, or if the code was modified directly by the client. Considering that you've just installed next.js the latter is the only possibility

And usually the html code modified by the client is caused by an extension or some experimental feature you have in your browser

Thank you very much

Np