Gmail was on an HTML5 roll this spring, adding drag-and-drop attachments, image insertion, and windows staying open after Gmail closes. Those features were Chrome-only, but have now made their way into Safari 5 on Windows and Mac. Presumably, any browser that supports these kinds of HTML5 features could get such support in the future—Firefox, Opera, and (from what we’ve seen) Internet Explorer 9 included.

HTML5 features now in Safari too [Official Gmail Blog]