> 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
Try looking at the 'opacity' property and the 'rgba()' color values in
CSS3. I think you'll see that CSS development is going in the right
direction on opacity/transparency.
Note that you can tell the CSS validator to check stylesheets against
CSS3 by using the "more options" menu.