RDP is a service which works on a specific port & cannot be accessed via web browser unless and until you embed it using java and access it using web browser. You will need to use a third party utility to have it embedded with java to be accessible using a browser.
There is a scenario where RDP might be disabled on server, but you have NETBIOS network access, there are tools available that can enable RDP remotely. But it will not be via web in anyway.
please find below information You can use Remote Desktop Web Connection to connect to a remote computer over the Internet. To do this, the remote computer must be running Windows Server 2008, and Terminal Services Web Access (TS Web Access) must be installed. (For information about installing TS Web Access, go to Terminal Services Web Access (TS Web Access) on the Microsoft TechNet website.) Here's how to connect to a remote computer using a web connection: Open Internet Explorer or another web browser, and type the URL using the remote computer's name or IP address, followed by /tsweb/ On the Remote Desktop Web Connection logon screen, in Connect to or Server, type the server name (the remote computer name), and then click Connect. Type your user name and password, and then click OK. When you are ready to disconnect, log off and then close your browser.
If you have RDP access to your server and want to enable multiple RDP connections, do the following: Log into the server using Remote Desktop. In Windows 2008 R2 Click Start > Administrative Tools > Remote Desktop Services > Remote Desktop Session Host Configuration. In Windows 2008 Click Start > Administrative Tools > Terminal Services >Terminal Services Configuration. Double click Restrict Each User to a Single Session. Uncheck Restrict each user to a single session. Click OK.