I'm new to CSS. I am just won dering how safe it is to use .png files for images with transparency? As far as browser support goes...
The only browser that doesn't support alpha transparency in PNG is IE6. All the remaining major browsers support it. We can use some third party tools to work out the issue in IE6 like IEPNGFIX.
Keep in mind that, if you are using the semi-transparency for a link (anchor) element, and if they end up not being clickable in IE6, give the links "position: relative" in the CSS. It's a common bug and that's a common fix.