I mentioned a while ago that Google requires in their webmaster technical guidelines that sites should support If-Modified-Since HTTP headers, and the answer of some members was, that it is not obligatory.
But here is exactly what Google says:
Quote:
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Make sure your web server supports the If-Modified-Since HTTP header. This feature allows your web server to tell Google whether your content has changed since we last crawled your site. Supporting this feature saves you bandwidth and overhead.
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Source: Webmaster Guidelines
At some point I just gave up the discussion, since I already implemented those headers on my sites, and I realized that it did not make any sense to discuss any further.
But yesterday I was reading this
Google Webmaster Tools Warns Of Spikes in Bandwidth Fees and I thought of sharing these news.
The issue there is not directly about the implementation of that HTTP header, but looking into that a bit deeper, you will see the sense of it in this case, since the primary issue is
bandwidth.
To go a bit further, see the comments of the Google employee John Mu on this topic:
http://www.seroundtable.com/archives/017751.html#comment-1056547