Will your webpage be loading inside an app? If yes, then you could use How to get the device's IMEI/ESN programmatically in android? to get the same
Otherwise, if you are just loading the page inside the phone's browser, I am afraid it will not allow you to do access the IMEI
By the way, there are several other ways of authentication besides IMEI. Do you have any specific requirements to use only IMEI. If you post your authentication needs, we might be able to suggest alternative solutions.
