If you're a web developer and care about having snappy web sites you'll know about YSlow for Firebug. I managed to get a grade A (96) but I'm suspecting that there's a bug in the YSlow analysis.
Expires header is inferior to using
Cache-Control which my site was already using fine with headers like:
according to the latest documentation but YSlow kept going on about setting
Expires headers. I prefer
Cache-Control since you don't have to do any date formatting which eats a few excess CPU cycles. If anybody knows why it's a good idea to use both
Expires let me know.
A more worrying possible bug in YSlow is that it's date comparison on the
Expires header is wrong. I use the RFC1123 format for the
Expires header which uses GMT which means today that the output looks like it's one hour behind since we're (here) in BST (British summer time). Here are the different outputs:
`date` on command line Fri Aug 6 17:26:36 BST 2007 looking at my wrist watch 17:26 RFC 1123 Fri, 06 Aug 2007 16:26:36 GMT (RFC 822 Fri, 06 Aug 2007 17:26:36 +0100)
If I add 5 min to the RFC 1123 YSlow claims the date is not in the future which is incorrect. To get my A grade I set the
Expires future parameter to be 1 hour and 5 min ahead to not confuse YSlow's date comparison. But if YSlow gets it wrong perhaps HTTP accelerators (eg. Squid) and web browsers might get it wrong too.