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

<===> error-dart-sass
Error: expected ")".
  ,
2 |   filter: progid:DXImageTransform.Microsoft.Alpha(opacity=20\);
  |                                                               ^
  '
  input.scss 2:63  root stylesheet

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