What counts as written notice?

[Jason Bilbrey]

If a tenant in a month to month lease wants to terminate their lease in Wisconsin they need to provide “written” notice to the landlord at least 28 days before their next rent payment is due...

I just purchased a property and when I first visited a tenant they verbally they were thinking about moving out. The seller didn’t indicate they were aware.

If the tenant texted or emailed the previous owner, does that count as “written” notice or does it have to be in writing in the form of a letter?

What does the law consider “written”?

Note: The lease does not have any provisions indicating electronic notice is allowed.