This information is presented for the sake of completion and historical accuracy.
Don't use the <BASEFONT ...> tag.
It will make your life difficult. See Break the <FONT> Habit for an easy tutorial on styles. If you would like to learn how to set the font for the entire page see Setting the Font of the Whole Page.
<BASEFONT ...> sets the default size of fonts on the web page. For example, to set the default size as 7 you would use this code: