First, try omitting the quotes from 12 and 24. Worth a shot.
Second, it's better to do this in CSS. See also http://www.w3schools.com/css/css_font.asp . Here is an inline style for a table tag:
<table style='font-family:"Courier New", Courier, monospace; font-size:80%' ...>...</table>
Better still, use an external style sheet or a style tag near the top of your HTML document. See also http://www.w3schools.com/css/css_howto.asp .