1 in 4 people have some sort of vision impairment that makes reading on the web more difficult.[1]
Elastic layouts and coding using em
With the default text size on most major browsers set at 16px many people like to make their text larger to make it easier to read and I believe the number is set to go up as monitors get larger. Plus I’m sure that many people don’t realise they can increase the text size of a web page by holding ctrl + or selecting zoom via a menu.