The warning begins with “Googlebot Cannot Access CSS & JS on…” and then goes on to explain that that if you continue blocking these particular assets, your website will earn “suboptimal rankings” in Google. This is because Google isn’t able to fully understand your website.Although the suboptimal rankings warnings aren’t anything new, the Search Console notifications are.
In October 2014, Google had updated their webmaster guidelines, warning then that if you blocked CSS and JS, you will experience “suboptimal rankings.” These messages seem to be there to help drill this fact deep into everybody’s mind, making sure we all understand the severity of the situation if you’re blocking CSS and JS.
Here is a picture of the notification as received by Michael Gray:
If you’re one of the lucky ones to get this message, just follow the instructions found in the email to help diagnosis the issue and/or use the fetch and render tool found in the Search Console to see what Google is seeing.
By receiving this notification, don’t worry too much, as quite a few webmasters are getting this warning.