Based on my observations, google and probably many others are in love with websites that include the Etag and Last-modified headers along with the if-modified-since and if-none-match header checking because they contribute to higher speed browsing, however, back in the day, headers in use were namely Expires and Cache-control.
I read elsewhere that google chrome browser ignores cache-control sent by the server and uses a value of zero. Because of this, I'm going to stick with using Etag and Last-Modified headers.
My question is what are the first browsers and versions that support Etag, Last-modified, If-modified-since and If-none-match headers?
The reason why I ask is because my site for the most part is a site that displays photos but I don't want people with extremely old computers to throw them out and buy new ones just in order to view a set of pictures. I would rather build a very large set of happy guests instead.