This document gives a demonstration of what will happend when you try to combine classes, to see the classes defined, right-click and select "View source".
What I tried to achive was to combine classes without changing the original classes.
! Note how some of the combinations changes the classes unattended/unvanted !
! Also note that I have used several different classes to prove what is actually going on. !
What this document prooves is that you can not combine two classes and expect to get a new value or rather, get the value you intended :-(, sad but true.
So what do I do if I want to combine?
Use two tags: Where the outer(first) tag has the original class set and the inner(second) tag has the new class set!

First we show a span af each class:
Fonts are now 15pt (.normtxt)
Fonts are now 25pt (.bigtxt1)
Fonts are now 25pt (.bigtxt2)
Fonts are now 25pt (.bigtxt3)
Fonts are now 25pt (.bigtxt4)
Fonts are now 25pt (.bigtxt5)
Fonts are now 200% (.txt200pct)

Combine two classes on the span tag:
Fonts are now 200% (.bigtxt1 .txt150pct on tag)
Fonts are now 200% (.txt200pct .bigtxt1 on tag)

Combine two classes on the span tag and the class:
Fonts are now 200% (.bigtxt2 .bigtxt2x on tag,class)
Fonts are now 200% (.bigtxt3+.bigtxt3x on tag,class)
Fonts are now 200% (.bigtxt4x .bigtxt4 on tag,class)
Fonts are now 200% (.bigtxt5x+.bigtxt4 on tag,class)
Fonts are now 200% (.bigtxt6.bigtxt6x on tag,class)
Fonts are now 200% (.bigtxt7x.bigtxt7 on tag,class)

This is how the different classes suddently look after you have made a combine on the class. They should all be 25pt large but aren't:
Fonts are now 25pt (.bigtxt2x)
Fonts are now 25pt (.bigtxt3x)
Fonts are now 25pt (.bigtxt4x)
Fonts are now 25pt (.bigtxt5x)
Fonts are now 25pt (.bigtxt6x)
Fonts are now 25pt (.bigtxt7x)

Now we will try to span in a span:
Fonts are now 15pt (.normtxt) (span 1)
Fonts are now 200% (.txt200pct) (span 2)
Fonts are now 25pt (.bigtxt1) (span 1)
Fonts are now 200% (.txt200pct) (span 2)

As you probably have noticed by now, the last result is what I was looking for, horrible and waste of CPU power, both at client and server side and waste of tags.

Valid HTML 4.01!Congratulations, this document validates as HTML 4.01 Transitional!

I wan't to validate myself! and see the result

Valid CSS!Congratulations! This document validates as CSS version 2!

I wan't to validate myself! and see the result