In my product am using a modeless dialog box with “OK” and “Minimize” button. Where I can place these buttons on the Top Left or Top Right? If I place on the Top Left it shows am focused only on the Mac Users. Is there any standards?
The browser is a platform agnostic environment. You don't have to follow OS convention (although it would definitely aid UX if it did).
Saying that though, you can't beat a bit of convention! Looking at some frameworks and examples, they generally follow the Windows way.
Here's a jQuery UI dialogue for example:
controlAlignment-windows and then float them left or right accordingly, that way you get the best of both worlds.
...I think users are savvy enough to scan for the controls necessary to dismiss a dialogue as the X is pretty generic and easy to spot so I would just stick to the convention.