Capture and Store tags

            string s = "";
            int l = 0;
            var sb = new StringBuilder();
            for (int r = 0; r < s.Length + 1; r++)
            {
                if (r < s.Length)
                {
                    if (s[r] == '<')
                    {
                        l = 0;
                    }
                    else if (s[l] == '<' && s[r] == '>')
                    {
                        string tag = s.Substring(l, r - l + 1);

                        if (!IsKnownTag(tag))
                        {
                            sb.Append(tag);
                        }

                        l = r + 1;
                    }
                    else if (s[l] != '<')
                    {
                        sb.Append(s[r]);
                    }
                }
                else if (s[l] == '<')
                {
                    sb.Append(s.Substring(l, r - l));
                }
            }