c# - What does vary:accept-encoding mean?

Question

the google page speed addon informs me:

The following publicly cacheable, compressible resources should have a "Vary: Accept-Encoding" header:
//some .js and .css files

I don't understand what this means. I've already compressed these files like so:

if (encodings.Contains("gzip") || encodings == "*")
{
    app.Response.Filter = new GZipStream(baseStream, CompressionMode.Compress);
    app.Response.AppendHeader("Content-Encoding", "gzip");
}

And this all seems to work. Why is having Vary: Accept-Encoding necessary?

Answer 1

It is allowing the cache to serve up different cached versions of the page depending on whether or not the browser requests GZIP encoding or not. The vary header instructs the cache to store a different version of the page if there is any variation in the indicated header.

As things stand, there will be one (possibly compressed) copy of the page in cache. Say it is the compressed version: If somebody requests the resource but does not support gzip encoding, they'll be served the wrong content.