I am quite confused regarding PCI requirements. Say if I want to develop a mobile web browser based shopping portal, am I allowed to? Under the PCI requirements?
If it is a website available on the web for normal browsers that has been made to be responsive then the device being used to access it is irrelevant.
If you are creating your own payment portal then there are other mobile considerations (to make the user experience a nicer one), but if you are using 3rd party tools I know that PayPal have a mobile payments API.
The PCI requirements are about the devices processing the payments. If you use others people's services (and so their devices and networks) then they are going to do a lot of the work. However, once you've picked a third party to process the payments, then see what they say specifically about PCI. For example, here's what Paypal say.