Hi there,
I was wondering, if we can use HTML to capture the input from USB arduino.
I just got an arduino duemilanove. My plan is to write a processing program on the arduino,
then read the switch input from A0 using normal Browser.
Thanks in advance/ 
I just got an arduino duemilanove. My plan is to write a processing program on the arduino,
You'd have better luck writing an Arduino program for the Arduino.
then read the switch input from A0 using normal Browser.
How is the browser going to communicate with the Arduino? There needs to be some logical and physical connection between the two.
Typically, this is done with an ethernet shield on the Arduino, and an ethernet cable connecting the shield to a router or switch.
Do you have an ethernet shield?
HTML is a text markup language, not a communications protocol, so, no you can use html to capture output from the Arduino. You can make the Arduino output text that is compliant with the html specification and that contains the sensor data of interest. That text is output via an ethernet shield/library/connection in response to a client (browser) request.
If you don't understand this, though, your chances of success are pretty low. The more of the process you understand, the better your chances are.