Here are the W3C recommendations

Declare your styles and create your classes.
Assign your selectors ( common HTML tags ) in your styles declaration.

Use contextual selectors when you want to apply a specific style.
Two selectors (HTML tags) seperated by a white space. The first specifies the context, the second selector will be the one to have its style properties changed.

Declare a class without an associative element.

<STYLE TYPE="text/css">
.class-name1 { style-definition }
b.class-name1 { style-definition }
</STYLE>

Creates a definition for all elements and then specifically for the <B> tag.

This over my head Tom, but can be found at W3C

regards,