Yesterday, Google sent mass notifications for blocked JavaScript and CSS. I recommend you read that story if you haven't yet.
Since then, there have been many questions about what to do. I recommended yesterday to unblock your CSS and JavaScript files, use the Fetch and Render tool to check those issues and check the email you received from Google for more details.
But Google is sharing more information now.
How To Quickly Unblock JavaScript & CSS Assets
Gary Illyes from Google posted on Stack Overflow the cheat, or quick way, of unblocking your JavaScript and CSS files from Google. Gary said the "simplest form of allow rule to allow crawling javascript and css resources" is to add this to your robots.txt file:
User-Agent: Googlebot
Allow: .js
Allow: .css
Gary said this will open it all up for GoogleBot.
Google Checks Your Home Page & Mobile View
Primarily when Google checks for blocked CSS and JavaScript assets, they don't go too deep into your site. They look mostly at just your home page and then the mobile/smartphone view of your web site.
John Mueller of Google said this in a comment on his own post on Google+ saying "'re primarily looking at the site's homepage & for the smartphone view of the page."
Google Doesn't Look At 3rd Party Embeds
John Mueller of Google also said there that you shouldn't get this notification from Google if it is a third party embed (ad code, social embeds, etc) that has blocked JS or CSS. You will see these warnings in Google Search Console, but you should not have received an email from Google for 3rd party issues.
John wrote in that Google+ post:
We're looking for local, embedded, blocked JS & CSS. So it would be for URLs that you can "allow" in your robots.txt, not something on other people's sites (though blocked content on other sites can cause problems too, eg, if you're using a JS framework that's hosted on a blocked URL).
How Many Webmasters Received This Notification?
I asked Gary Illyes from Google about how many people received this notification. I didn't think he would answer, but he did shed some light on it.
He said on Twitter that Google sent out 18.7% of what they sent out for the mobile usability issues. So you thought this JS and CSS notification was sent out to a ton of people? The mobile usability notification was sent out to almost 85% more webmasters.
Forum discussion at Stack Overflow, Google+ and Twitter.
