I first took a close look at this in early 2015, when I made some changes to the website so it would display better on mobile screens. At that time, 39% of the visits to the ATC websites were from mobile devices (phones), 49% were still coming from computers, and tablets were at a bit of a peak—12% of the visits were on tablets.

plot of percentage of visitors to ATC sites by Desktop, mobile, or tablet

Now, for the year ended on 22 April 2020, 68% of the visits are from mobile devices.