.youtube_videowrap{
  width: 100%;
}
.youtube_videowrap .ytb_cl6{
  margin: 10px 10px;
  display: contents;
}
.youtube_videowrap .ytb_cl6 iframe{
  width: 49%;
  height: 300px;
}
@media (max-width: 768px) {
  .youtube_videowrap .ytb_cl6 iframe{
    width: 100%;
    height: 300px;
  }
}