Hey Lance,
There are certainly areas where you do have to tailor an application or site to meet particular nuances of a specific device or mobile OS, but in the majority of cases the approach you apply for one device can be applied to another also. Things like varying device screen size and resolution (320x480 portrait is the roughly the size you expect in a standard smartphone) and less capable mobile browsers do throw a spanner in the works though :/
I've tried to answer a similar question in another approach, so if you have the time I'd recommend reading my answer there (I think that
thread is going to be quite active).
https://coderanch.com/t/524844/Android/Mobile/Android-web-app-other-devices
Cheers,
Damon.