No. While you can set a base font size on body using the font-size property, anything after that that specifies a smaller size will override the base rule for that element. In order to do what you are looking to do you will need to use Javascript.
You could iterate through the elements on the page and change the smaller fonts using something like this:
$("*").each( function () {
var $this = $(this);
if (parseInt($this.css("fontSize")) < 12) {
$this.css({ "font-size": "12px" });
}
});
Here is a Fiddle where you can see it done: http://jsfiddle.net/mifi79/LfdL8/2/