Robert_Charlton - 6:41 am on May 2, 2013 (gmt 0)
To further clarify when "The Vary HTTP Header" applies, looking at it from a different angle. Google is not necessarily recommending it for all mobile sites.
The article partially quoted in the opening post here is a Google Developers >> Webmasters article...
Building Smartphone-Optimized Websites
Last updated April 12, 2013
The article begins...
Google supports smartphone-optimized sites in three configurations
1 - Google's recommended configuration is "responsive web design... sites using the same set of URLs with the same HTML to all devices... and using CSS to change rendering.
2 - Sites dynamically serving all devices on the same set of URLs, but "each URL" serves different HTML and CSS, depending on the user agent. This is the configuration for which Vary HTTP header applies.
3 - Sites having separate mobile and desktop URLs.
Under "Dynamically serving different HTML on the same URL", the article then goes on to highlight the two important implications of the Vary HTTP header, which are that...
1 - It signals caching servers used in ISPs and elsewhere to consider the user agent...
2 - It helps signal to Google that alternate mobile content is being served.
So, be aware that not every site configuration needs to be concerned about the Vary HTTP header. That said, as large screen mobile hardware becomes available... larger smart phones, tablets, convertible laptops, etc... viewport size becomes a key consideration, and there is ongoing discussion about whether responsive design or alternate content might be preferable.
Recent discussions (to choose among several)....
Checklist for Mobile Site Optimization?
Dec 26, 2012
How Do We Do This?
Want to take a version of our PC site to Mobile
Dec 3, 2012
Responsive Design versus separate site
Feb 24, 2013