So, although you can just use the excellent Microsoft To-Do UWP application, for completeness here's the web application running under Chrome on my Mac:
To-Do is turning into quite a stunning piece of cross-platform work. Update or edit anywhere and the changes are available everywhere. Perhaps the cornerstone of your work or family life? I'm sure you can think laterally and extend where this might be used?
The URL to go to in your borrowed or emergency browser is to-do.microsoft.com
PS. As I say, I tried this on the Lumia 1020, but the small screen foxed me - anyone still got a Lumia 1520 on 8.1 and willing to try this? - I think it might well work, giving a theoretical way to get Microsoft To-Do on the older platform.