Did a quick google search and came across this:

http://www.walking-productions.com/jsserial/new/JSSerial.html

Guess you can via JavaScript.
-Josh

On Tue, Jun 21, 2011 at 9:27 AM, alan smith <micro_eng2@yahoo.com> wrote:
> Are there any extensions of HTML to send commands thru the serial port of=
 the PC from a browser window? =A0I was asked this, and thought....maybe? =
=A0This guy wants to create buttons, etc and use it to send serial commands=
 to some hardware for setup and diagnostics. =A0I told him VB can do this, =
but he isnt a hard core programmer but can do HTML thru the many WYSIWUG pr=
ograms.
> --
> http://www.piclist.com PIC/SX FAQ & list archive
> View/change your membership options at
> http://mailman.mit.edu/mailman/listinfo/piclist
>

--=20
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist
.