<===> input.scss
test {
  filter: progid:DXImageTransform.Microsoft.Alpha(opacity=20\));
}

<===> error
Error: Invalid CSS after "...ha(opacity=20\)": expected ";", was ");"
        on line 2 of input.scss
  Use --trace for backtrace.

<===> output-dart-sass.css
test {
  filter: progid:DXImageTransform.Microsoft.Alpha(opacity=20\));
}