Had a quick look at Mercury OS, to me it looks like a mock-up, not an implementation.<p>For me the term "OS" refers something along the lines of Linux, BSD, Minix, Haiku-OS, Minix, Plan9, etc. For each of these a large amount of effort has been, and continues to be, expended on optimizing for CPU architecture, device drivers, window managers, etc.<p>If you are more interested in <i>operating environments</i> and experimenting with presentation and manipulation concepts, then as you have hinted, using the web browser environment as a virtual machine is a far more productive approach to trying out different concepts without massive amounts of very low level programming.<p>I rather like the look of Self and Smalltalk environments. Have toyed with some similar concepts running in a browser in full-screen and/or kiosk modes.