Getting started with practical web accessibility, section 2 Demonstration: an analysis of a page:

Rule 10: Font size

Accessing the page, we can see that copy text appears in a font size considerably smaller than the expected 16 pixels. It is not necessary for the analysis to know the exact size, but it is 11 pixels. (This can be seen by looking at the page’s HTML source code, finding the reference to the style sheet, and viewing the style sheet.) This typically corresponds to about 8 points. Compare this with the fact that on paper, 10–12 points is often cited as required for good legibility. On screen, legibility is generally worse than on paper, since the screen has much lower resolution and stability

Almost all text on the page remains in the small font size even if the user sets the browser’s basic font size via the View > Text size. Thus, the font size issue is serious.

The font that the page’s style sheet suggests is Verdana. It is relatively readable even in small font sizes, so this somewhat alleviates the problem. However, in browsing situations where Verdana is not available, the problem becomes worse.