cellspacing and cellpadding in percentage lengths

I'm not sure if I should be sending this question to
you. If not, I apologize for taking up your time and I
hope you could direct me to the right place.

According to the HTML 4.01 spec, TABLE attributes
cellspacing and cellpadding can be set as percentage
lengths. How are percentage lengths applied to these 
two attributes? 

For example, if I wrote

<TABLE cellspacing="20%">

which "available space" is the 20% calculated 
with respect to? 

Thanks,  Irene.