What happens to Windows 10 login should MS servers go down?

Hypothetical question but logging into Windows 10 is heavily reliant on the MS account and I wonder what would occur if MS servers were down (or maybe your own ISP is having problems)?

We always get met with a login screen as we have several family users (all MS accounts as opposed to local). I presume on login there is some kind of handshake/authentication with MS servers? But what happens if the MS server was down for any reason? I assume it would still login ok and in effect act as a local account - would the login pause/stay on 'logging in' for a set time waiting for a MS response before timing out after a set period?

it only got me thinking because my wife logged in earlier. Normally after a couple of seconds she's on the desktop but earlier on it took a bit longer on the 'logging in' phase - it wasn't anything to write home about, say 10 - 20 secs. But it got me wondering whether on this occasion she was just the victim of a sluggish MS server response or something like that. We did test again a bit later on and she was back to logging in within a second or two.

An easy test is temporarily disable/disconnect your network connection and see how everything works.
For me, it's more likely my ISP service will go down, and i don't have any internet connection possible.

As a backup precaution, create a Local Admin account.
Then you can login in with that if something happens to MS servers/accounts and no-one can login with a MS account.
You can have it PW protected where only you know the PW, so you can do whatever needed in case of emergency.

Ok thanks to you both, yes I do have a local admin account that we never use as another way in.

i just wondered if anyone had any experience of what happens - I was hesitant to test the scenario in case it had any side effects. I might take a recent system image first before trying this out.

Periodically log in with the Local account to make sure it still works.

I had a situation once where a MS account was getting errors and warnings.
The local account worked without any problems.
It was a pita to fix the MS account, it took me several hours.
That was with 8.1, but such things can happen.
I agree, backup images are important and i do them regularly.
Unfortunately, restoring a backup image would not fix a problem with a MS account (for me).

Turned out it is a bug, and the MS account had to be "fixed".