Google Search is planning to launch SXG support for desktop users in the coming weeks.
Sites using dynamic serving
(varying by User-Agent header) will need to annotate their pages as mobile- or desktop-only, as documented here
. For example:
<meta name=supported-media content="only screen and (max-width: 640px)">
Otherwise, desktop users may see the mobile version of the page.
We're reaching out individually to SXG sites who we've found to use dynamic serving, but I wanted a broad announcement in case there are any that we miss.