2.1.36 [HTML] Section 15.2.2, Font modifier elements: FONT and BASEFONT
The BASEFONT element sets the base font size (using the size attribute).
IE7 Mode, IE8 Mode, and IE9 Mode (All Versions)
The BASEFONT element is defined to be empty in the DTD http://www.w3.org/TR/html401/sgml/loosedtd.html#basefont . In IE9 Mode, the element is not treated as empty.
BASEFONT elements are treated in a similar way to b and i elements when parsing. Additional BASEFONT elements might be added to the tree as a consequence of this parsing behavior.
The base font size does not apply to headings, except where these are modified using the FONT element with a relative font size change.
Quirks Mode and IE7 Mode (All Versions)
The base font size does not apply to headings, even when the base font size is modified using the relative font size change on the FONT element.
IE8 Mode and IE9 Mode (All Versions)
Both the base font size and the relative font size change on the FONT element apply to headings.