public IEnumerable<CssRule> Rewrite(CssRule rule) { var declaration = rule.GetDeclaration("opacity"); if (declaration == null) { yield return rule; yield break; } var value = declaration.Value as CssNumber; if (value == null) { yield return rule; yield break; } var index = rule.IndexOf(declaration); // Add the filter before the standard rule.Insert(index, new CssDeclaration("filter", $"alpha(opacity={value})")); yield return rule; }