Web browsers have a default configuration after the installation. However, the standard font sizes differ between the different browser types and releases. In addition to that, the real size of the characters displayed by browsers also depends on the screen resolution and pixel size. Therefore, you cannot predict the actual font size.
Nevertheless, users have different preferences concerning font sizes. If users with visual disabilities have set the font size in their browsers on a higher value, they expect to read text in bigger font sizes than usual.
Therefore, Nielsen recommends non-fixed font sizes and to ensure that the design works well with different font sizes. You can easily check it by varying the common font size in your browser.
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