I've always been unsure of where to start as a general best practice baseline. Yes, I know it depends on your design, but what's most common?
Here's what I currently have as a starter:
h1 { font-size: 24px;}
h2 { font-size: 22px;}
h3 { font-size: 18px;}
h4 { font-size: 16px;}
h5 { font-size: 12px;}
h6 { font-size: 10px;}
Yes, we don't use EMs at my current job.
Headings are normally bold-faced; that has been turned off for this demonstration of size correspondence. MSIE and Opera interpret these sizes the same, but note that Gecko browsers and Chrome interpret Heading 6 as 11 pixels instead of 10 pixels/font size 1, and Heading 3 as 19 pixels instead of 18 pixels/font size 4 (though it's difficult to tell the difference even in a direct comparison and impossible in use). It seems Gecko also limits text to no smaller than 10 pixels.
Source: Stackoverflow.com