Transparent Glassy Header special CSS not working

A few years ago I was given this Glassy Header CSS code. And used it on other sites and it worked.

However now I get an ! type error on filter:

This is the CSS code

#top .av_header_glassy.av_header_transparency .header_bg {
background-color: #fff;
opacity: 0.5;
filter: alpha(opacity=50);
}

However in Appearance > Theme Editor > where I do the editing for the Child Theme, it has an ! make on the line for filter:
This is what it says on mouse over…
“! Expected (<filter-function-list> | none) but found ‘alpha(opacity=50);”

Your help would be greatly appreciated.

Cheers Dave