Optional NFC login to web based system
The progress of log in: The user starts the browser and go to your website and instead of manually entering credentials he clicks on the "log in via NFC." Server retains identification session from that IP and date (and maybe other info about client hardware for safe) to the database and "expects" NFC incoming data.
On the client PC / Phone you'll have to install your application/service, which will be able to receive data from the NFC scanner (who usually works as a keyboard) and sends them to your server, eg. Via ASP.NET WebAPI or other REST ...
The server will accept data from the IP and find a record in database of that IP perform log in (+ a time limit? + checking client hardware for safe?). Then the server side you have confirmed logon and the user can proceed (you can redirect him to our secure site).
Note. 1 The critical point is to pair the correct and safe Identification client browser and PC/mobile application which reads NFC tags.
Note. 2 You will need to select the appropriate NFC scanner, which will ideally have a standardized drivers built-in Win / Linux OS (otherwise you often solve the problem of missing / non-functional NFC drivers).
I have a web system where staff can log in with a username and password, then enter data. Is there a way to add the option for users to seamlessly log in just by swiping the card against an NFC scanner? The idea is to have multiple communal PCs people can walk up to and quickly authenticate. 
It's important that the usual text login form works too for people using the site on PCs or phones without the NFC option.
The web client PCs with an NFC scanner could be linux or windows.
(The web system is a bootstrap/jquery site which gets supplied with JSON data from a python web.py backend. I'm able to modify the server and the client PCs.)