On IE11 I am shown a list of two possible logins. One is my Office 365 enterprise login and one is my personal Microsoft account login. I am using W10 Pro and both accounts are associated with my PC user ID.
On other browsers, I am only shown the Office 365 login.
In addition, this site ignores any existing sessions I have in O365 or outlook.com and forces me to select a login each time.
Finally, it isn't clear how this site will choose between possible logins. This issue is created due to the artificial separation between O365 and Microsoft accounts. Many of us have been forced to maintain multiple Microsoft accounts and possibly multiple O365 accounts too. This is especially true in the IT Pro arena as we are likely to have personal (consultancy) accounts and customer accounts.