There is no attribute "allowtransparency"

I am looking for some alternate way to do:

<iframe transparency=true   ...

When I do the W3C validation, I am getting the error:

Column 31: there is no attribute "allowtransparency"

If use this CSS,

.iframe-trans-fix{
   opacity: 0;
   filter:alpha(opacity=0);
}

for the above snippet I am getting a blank iframe.