Browser ignore Cache-Control

I set a static file on Amazon S3 to use Cache-Control: max-age: 259200, must-revalidat开发者_如何学Pythone. However, when I load a test page on my browser in Safari, the resource tracker shows it's getting the file from Amazon S3 even though it's supposed to be cached. What's going on? Do browsers have special caching rules I'm not aware of that override Cache-Control?