How does web page send data to port of computer

Hello

I have computer that has internet connection and Microcontroller is connected to computer via usb to db9 cable. LED is connected to port pin of microcontroller

Computer -> web page (com port of PC)----- DB9 connector -> max Rs232 - > micro c -> LED

I want to control LED via web page I have created a web page , there are two buttons on web page , start and stop, if I press start button on web page , led should be “on’’ and If I press stop button on web page ,led should be turn...

How does web page send data to port of computer