Hi Maskkkk. You've noted that your router has logged an entry relating to port 80. That probably confirms that your ISP is not the issue here. You will need to confirm that the entry is on the "allowed connections" log as opposed to the "denied connections" log. If you can confirm this, we will then need to look at webserver machine itself. You mentioned that you have a firewall installed on the webserver machine. Are you sure that the iptables rules allow both incoming port 80 connections as well as outgoing connections from your webserver (from any ports).
Lastly, have you tried accessing your webserver from any other external locations i.e. friends, home, work, etc. This is to eliminate any specific problems between W3C's HTML validator and your webserver.