You absolutely have to have some sort of server-side scripting here - to store the messages on the server - otherwise they would only exist in your own client window, and disappear forever when you closed it.
You probably only need a simple script which dumps each new message into a new file, and another which reads in the files to show your message board. The most complicated part would be making sure you read the files in in the correct order - you would need to organise a sequential naming scheme, or read them in according to age, oldest first.
You could also use just one file - it would be displayed more quickly, but then you have to worry about file-locking so that 2 people don't add messages at the same time and you end up with garbled shite / corrupt XML.
It's not such a huge project, but if you can't be bothered, go have a look at Hotscripts.