Since version 1.7 the Web Viewer runs locally on all major browser (tested in Chrome, Firefox, Safari, IE). Please update to 1.7. In case you are not able to update:

The Antetype web viewer works by default using the local file system to pass the files to a web browser - usually Safari.

However recent upgrades to Safari and Chrome mean that the web viewer does not always work from the file system.

There are a few ways to solve this problem:

1. In Safari enable local files: From the Preferences Panel in Safari Choose the Advanced Tab, then make sure there is a tick in the Show Develop menu in menu bar.

From the Develop menu tick "Disable Local File Restriction"

2. Run a local simple web server in the web viewer folder: You can easily start a simple web server from the OS X Terminal prompt using the following command:

python -m SimpleHTTPServer

There is an OS X Daily Article with more information.

3. Use web sharing and the Sites folder to run a local web server: Prior to Mountain Lion this could be enabled in the System Preferences -> Sharing - Web Sharing.

For mountain lion users who do not have the Web Sharing option please read this article on CNet

4. Use Firefox - Firefox currently doesn't have this restriction in place.

5. Host the files on your own web server: Accessing the files from any web server will get around this restriction in Chrome and Safari.