if private key is saved in users browser cache than if we clean cache than we will lost our access to coin? 
Hi,
you can use backup feature to gain access to your wallet at any time from anywhere
see at: Security -> Backup/Restore wallet
For tests you can use my private testnet key: cPtyQjUSvdkWV2ahPVtKajHEhARc2BavfaokHwXPaoGMdmKMP4gT
Than i think you can also add lots of other coin in your site in this way.