For a demo of Live.js we allow you to temporarily change the style.css file. So, have the homepage and this one open and visible. Mess up the CSS and look at the homepage.
Your changes are saved to the /style.css file included on both pages. Saving is not part of live.js ;-) But as Live.js is also loaded, it applies your changes without a reload.
And that's not all. Live.js also applies changes to HTML or Javascript. So include it in your own project right now and forget about Alt-Tab.
Or use the bookmarklet to only monitor CSS changes on any page you're working on! Drag the following link to your bookmarks bar: Live.js