It doesn't require it, but it recommends it: "To mark a response as 'never expires,' an origin server sends an Expires date approximately one year from the tim

It doesn't require it, but it recommends it: "To mark a response as 'never expires,' an origin server sends an Expires date approximately one year from the time the response is sent. HTTP/1.1 servers SHOULD NOT send Expires dates more than one year in the future." -- http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html In practice, far-future expiration headers used by Yahoo!, Google, and others are often well over a year in the future, and all browsers support this just fine. I did do some sanity checks using various dates in the headers, and I saw no difference between dates of less than a year and dates over a year in the future. I decided to run the canonical tests using the far-future dates since that's what the majority of websites use these days.