Today, most people are searching on Google
using a mobile device. However, our
ranking systems still typically look at the
desktop version of a page’s content to
evaluate its relevance to the user. This can
cause issues when the mobile page has less
content than the desktop page because our
algorithms are not evaluating the actual
page that is seen by a mobile searcher.



To make our results more useful, we’ ve
begun experiments to make our index
mobile-first. Although our search index will
continue to be a single index of websites
and apps, our algorithms will eventually
primarily use the mobile version of a site’s
content to rank pages from that site, to
understand structured data, and to show
snippets from those pages in our results. Of
course, while our index will be built from
mobile documents, we’re going to continue
to build a great search experience for all
users, whether they come from mobile or
desktop devices.



We understand this is an important shift in
our indexing and it’s one we take
seriously. We’ll continue to carefully
experiment over the coming months on a
small scale and we’ll ramp up this change
when we’re confident that we have a great
user experience. Though we’re only
beginning this process, here are a few
recommendations to help webmasters
prepare as we move towards a more
mobile-focused index.



If you have a responsive site or a
dynamic serving site where the
primary content and markup is
equivalent across mobile and desktop,
you shouldn’t have to change
anything.



If you have a site configuration where
the primary content and markup is
different across mobile and desktop,
you should consider making some
changes to your site.



Make sure to serve structured
markup for both the desktop and
mobile version.



Sites can verify the equivalence
of their structured markup
across desktop and mobile by
typing the URLs of both versions
into the Structured Data Testing
Tool and comparing the output.
When adding structured data to
a mobile site, avoid adding large
amounts of markup that isn’t
relevant to the specific
information content of each
document.



Use the robots.txt testing tool to
verify that your mobile version is
accessible to Googlebot.
Sites do not have to make
changes to their canonical links ;



we’ll continue to use these links
as guides to serve the
appropriate results to a user
searching on desktop or mobile.
If you are a site owner who has only
verified their desktop site in Search
Console, please add and verify your
mobile version .



If you only have a desktop site, we’ll
continue to index your desktop site
just fine, even if we’re using a mobile
user agent to view your site.



If you are building a mobile version of
your site, keep in mind that a
functional desktop-oriented site can
be better than a broken or incomplete
mobile version of the site. It’s better
for you to build up your mobile site
and launch it when ready.
Official Google Webmaster Blog


My opinion

I think website with different
mobile and desktop version should really
buckle up if you want to receive organic
traffic and rank your blog higher with
Google.

http://naijalane.com.ng/2016/11/09/googleannouncesmobile-first-indexing/

cc lalasticlala, seun

OK

Thank you for the wonderful insight. So we can eventually break down this update to the following:-

IF Responsive website + Same Content and Markup (Mobile Version +Desktop Version)

-No need to change anything

IF Separate websites + Different Content and Markup (Mobile Version +Desktop Version)

-Make sure structured markup across both websites
-Ensure that the robots.txt for the mobile website is available to Google bot (Robots.txt tester tool)
-Make no changes to the canonical links

IF Separate websites + Same Content and Markup (Mobile Version + Desktop Versio)

-Check if both the versions have been added and verified in Google Search Console

IF Only a single desktop website

-Google will still index your website just fine
-However, for accessibility and UX reasons, it is recommended to have a mobile version ==> More hits

All this because Google search algorithms are being updated to give preference to the mobile version of a page to evaluate its position in the search results.

Thanks,

References:
http://www.zealousys.com/blog/mobile-first-indexing-relevant-practices/