While games written with Cocos2d-html5 should work offline, but some browsers won't allow this to happen. Browsers that deny access to certain functions such as XMLHttpRequest that fails for "file:// " protocol, but Cocos2d-html5 engine depend on this to read many files such as a .plist file.
Some versions of Firefox, Safari and Opera are notable exceptions.
If you wish to use other browsers, you need a webserver. It doesn't mean a seperate computer, you can download a free webserver program to your computer. Here is some popular webserver program:
Once you install a webserver, go to your installation directory, there should be a directory called "htdocs" or "www", place Cocos2d-html5 files in there. Then point your browser to http://localhost/.Cocos2d-html5 should now work without any errors