With FFFFALLBACK, you can easily test fallback fonts for a webpage. You can easily see the combinations of alternate fonts and finally choose combinations in such a way that you designs don’t look ugly even if the primary fonts aren’t available on the user’s computer. FFFFALLBACK is a browser bookmarklet and thus easy to install. Just drag this bookmarklet link to your bookmarks toolbar and it will be installed instantly.
After installing it, go to any webpage and click on the bookmarklet. FFFFALLBACK then scans the webpage to find the existing web fonts used. It then clones the page and provides you a sidebar with a handful of tools that you can use to test various fallback font combinations on the page.
The whole idea behind FFFALLBACK font testing tool or bookmarklet was inspired by a blog post on Choosing Fallback Fonts.