to: 厚道人

what is the 它 in 網絡瀏覽器隻允許一個小應用程序在給它提供服務的主機讀取和寫入數據, is 網絡瀏覽器 or 小應用程序?
what is 套接字?

once the applet downloaded, it has nothing to do with the Server (Webserver, or the Host, where the Webserver is).

Applet can use io of the client machine (where the 網絡瀏覽器 is), only if the client allows it, see class SecurityManager and google Applet Security Manager