In the beginning, majority websites came up with small fonts as 12 pt, but with time people understood that the fonts are giving a tough time to go through online. Especially when the screen is just 24 inches, it becomes a big struggle to read the smaller fonts. The thumb rule is to see on the web and to maintain at least 16 pt.
The number alone does not ensure readability in a website design. This is because it also depends on the font. For online texts you may consider the Times New Roman that belongs to the Serif font’s family.