Little issue about CSS standard

Hello, my name is Javier, I'm from Argentina, and I have a question  
for you.. I'm developing a web site, and I'd like to make a valid CSS  
code, as XHTML that works for all browsers. The problem, is that I  
want to use alpha tranparency or opacity respectively and I found that  
they are both not valid  CSS attributes, the question is: Why if these  
attributes exists, and works when I use them, they are not included as  
valid CSS attributes on W3C CSS standard? Anyway, if you are trying to  
validate a CSS standard, and I add CSS attributes to my XHTML code  
using JavaScript once html content is loaded, you cannot validate  
them, and I'm making valid CSS for your CSS server side validator  
tool, but, really I'm supposedly not, because I'm adding supposedly  
"not valid" CSS attributes to my XHTML DOM objects by using client  
side JavaScript once page is loaded, that, off course works for all  
navigators, but you cannot see them beacause they are generated by a  
client side script. So you are making a wrong validation and I'm  
adding more download time to my pages just for fix this problem. I  
hope you guys fix this issue when you release the CSS3 standard. It is  
not about to use a transparent PNG to make the same effect, because  
explorer under its 7th version has no support for this, and 98 % of  
Internet users, has Explorer running on their PCs, but anyway, you  
know that exists little hack for this, and the whole world use it, It  
works, but you consider it as not valid, and that is the moment I  
which I say What's happening?. So, if you don't fix it, well, I'll  
have to made my own jQuery CSS support to avoid all these "wrong CSS  
attributes", that off course you wont be able to validate, but this is  
the Web that you're making... I think.

Thank you very much.

Javier.

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