Welcome to LinuxQuestions! I hope I can make a good first impression by fixing your issue on the first try. Congratulations on choosing Slackware as your distro of choice. If you learn to use Linux on Slack, you'll be able to handle pretty much any other distro out there, so stick with it. I think I know what your issue is. You never mentioned your file permissions.
Are your file and folder permissions setup right?
You have to make sure your public html directory (and files inside it) are able to be "read" by anyone. Break out a console and lets see if we can get this bad Jackson rolling.
$>chmod 755 *your public html folder*
$>cd *your public html folder*
$>chmod 755 index.html
That should fix it. People new to Linux that have migrated from Windows usually forget about having to setup their file permissions correctly. Don't feel bad, I stumbled around for an hour during my newbie years trying to find out what was wrong. Turns out I had my "htdocs" directory permissioned wrong.
Let me know if this works or not.