Assertions:
The effect of BASEFONT SIZE in Internet Explorer 1.0-2.0, Netscape 1.0 and Opera is to serve as a starting point in calculating relative FONT SIZE change values. Later versions of Internet Explorer and Netscape 1.1-4.x allow the setting of a default font size for contained text.

Normal relative font size changes -3 -> +3, with no BASEFONT modification:

Filler text
Font size=-3
Font size=-2
Font size=-1
Font size=+0
Font size=+1
Font size=+2
Font size=+3
Filler text

BASEFONT SIZE=6 encapsulating relative font size changes -3 -> +3:

Filler text
basefont size=6
Font size=-3
Font size=-2
Font size=-1
Font size=+0
Font size=+1
Font size=+2
Font size=+3
basefont size=6
Filler text