In 2015 web access from mobile browsers (smart phones and tablets) was approaching 50% of web page accesses.
In 2015 Google posted on the official Webmaster Central Blog,
that they would be extending the use of mobile-friendliness as a ranking signal.
It would affect rankings on mobile devices only.
Wide pages with things like tables and long performed text (<PRE>) strings will display on mobile devices but will be reduced to fit on the screen, so hard to read. You can enlarge the view and then scroll left or right to see it, but google does not consider this mobile friendly.
They will downgrade your ranking when you search for a page that is not mobile friendly.
Early mobile phones had around 320 x 480 displays.
A page enclosed in a table with width="100%" was mobile-friendly.
limit for preformated text seems to be around 64 characters in 10 point font.
1234567891123456789212345678931234567894123456789512345678961234 0 0 0 0 0 0 </PRE>Preformatted text can be up to 64 characters in 10 pt font.
510 pixels on my MacBook Pro and my Samsung galaxy tab S3, 1,000 pixels on my iPhone 5s, 695 pixels on my iPhone 7+.
See a larger PRE
Tables Tables up to 500 px are mobile-friendly
A non friendly table which was designed for 1,000 pixel width. - Denver Hiking
How to make wider tables mobile friendly:
Example from PCMag.com
Things to do to make mobile friendly pages:
Mobile-Friendly Test - Google Search Console
Mobilegeddon: A Complete Guide to Google's Mobile-Friendly Update
How to pass the Google Mobile-Friendly Test, for legacy sites - LucarInfo Blog — LucarInfo Blog
mydevice.io know your mobile device (pixel-ratio, CSS width, features)
Responsive Web Design Basics | Web Fundamentals | Google Developers
Creating a mobile-friendly web page with Dreamweaver CS5.5 - Tutorial - Dreamweaver Club
Dave's Mobile-Friendly notes