CSS - Inherited attributes

This is probably a pretty basic question, but I haven't seen it mentioned in any of the CSS tutorials I've read.

If I have

table.base td
{
width: 50%;
border: 1px solid black;
}

and this is my base style that I want for all of my tables tds. Now I want to make multiple children table tds styles that all include the previous styles included in table.base. So instead of writing

table.child1 td
{
width: 50%;
border: 1px solid black;
background-color: #999999;
}

I only need to add the background-color.

Thanks,
Anthony