Partially Sighted and Visual Acuity
Magnification, the use of high contrast text and alternative background colours may be important to many users with visual impairments. The way content on a web page is laid out with reflow when zoomed, also affects ease of reading. Too much clutter on a page can cause overlapping of elements as well as text. Screen-reading may also be used as scrolling horizontally when text is very large can be very tiring. Further advice from an Envato tutorial on Accessibility Basics: Designing for Visual Impairment.
These tests contribute towards accessibility scores for this disability.