MinifyHtmlAttribute.cs

[AttributeUsage(AttributeTargets.Class | AttributeTargets.Method)]
    public sealed class MinifyHtmlAttribute : ActionFilterAttribute
    {
        public override void OnActionExecuted(ActionExecutedContext filterContext)
        {
            if (IsSupportedContentType(filterContext))
            {
                filterContext.HttpContext.Response.Filter = new MinifyHtmlStream(filterContext.HttpContext.Response.Filter);
            }
        }

        private static bool IsSupportedContentType(ControllerContext filterContext)
        {
            return filterContext.HttpContext.Response.Filter != null && filterContext.HttpContext.Response.ContentType.Equals("text/html", StringComparison.OrdinalIgnoreCase);
        }
    }
    
public sealed class MinifyHtmlStream : MemoryStream
    {
        private static readonly Regex RegExpressionWhiteSpaces = new Regex(@"(?<=\s)\s+(?![^<>]*</pre>)", RegexOptions.Compiled);
        private readonly Stream responseStream;

        public MinifyHtmlStream(Stream responseStream)
        {
            this.responseStream = responseStream;
        }

        public override void Close()
        {
            var minified = Encoding.UTF8.GetBytes(RegExpressionWhiteSpaces.Replace(Encoding.UTF8.GetString(ToArray()), string.Empty));

            responseStream.Write(minified, 0, minified.Length);
            responseStream.Flush();

            base.Close();
        }
    }