Google may pick desktop over AMP page for the mobile-first index
Google's mobile-first index may index your desktop version over AMP when there is no mobile version of your website.
But one somewhat shocking item I learned yesterday was that Google, by default, won’t use the AMP version of your page for the mobile index, even if you do not have a mobile-friendly alternative.
For example, you have a website that is only desktop-friendly, i.e., there is no mobile-friendly version, but you decide to skip mobile-friendly and go directly to building AMP pages for your site. So now you have a desktop version and an AMP version of your website. AMP is super mobile-friendly, it is super-fast, and it is specifically designed for mobile.
In the case above, Google said they will not use the AMP version in the new mobile-first index; instead it will pick your desktop version. You can force Google to use AMP in this case, but you need to use a rel alternate attribute to force it. But by default, Google will pick your desktop version for the mobile-first index.
Jennifer Slegg reported that Maile Ohye from Google said this at the State of Search conference:
I found this to be a bit surprising, and I asked Gary Illyes from Google about this. He confirmed on Twitter:
He also added that he has not seen a case where a site had AMP and did not also have a mobile version:
Again, it seems weird that Google would pick the desktop version over AMP, but in many cases, with AMP, there is no real site navigation to crawl the website. So on the indexing side, that is why this might make sense.