Android Silent APK Install (no root)

Hi fellow Android Programmers. Our client is asking for this, and we hate to disappoint our customers. Some of you might say it's impossible. If that's your mindset, don't even bother bidding. What we need ? To specify; APK needs to be downloaded and installed silently, without root permission and without user interaction. this should work on any android device even without allowing unknown apps to install on device. How it should work ? via custom web page - download and install have to be done by only clicking on the link to custom web page. If you have experience in trying this out, let's have a talk.