The text-size-adjust property in CSS
Mobile browsers can render text using an entirely different rendering algorithm than desktop browsers at the browser / CSS level, to account for the fact that most websites were not originally designed for mobile use.
In Mozilla’s words:
Instead of laying out pages at the width of the device screen, they lay them out using a viewport that is much wider, usually 800 or 1000 pixels. To map the extra-wide layout back to the original device size, they either show only part of the whole render or scale the viewport down to fit.
Since text that has been scaled down to fit a mobile screen may be very small, many mobile browsers apply a text inflation algorithm to enlarge the text to make it more readable. When an element containing text uses 100% of the screen’s width, the algorithm increases its text size, but without modifying the layout. The text-size-adjust property allows web authors to disable or modify this behavior, as web pages designed with small screens in mind do not need it.
Read more at Mozilla
Some folks have been writing about this since … 2002!? WHO KNEW?