wonderful ! Thank you. It's new because it was not so before High Sierra...

Thank you for your attention.

So, on Safari : on Facebook there is the little green arrow at the top left of the videos, and the download of the videos is done well. On the other hand, on Youtube, this same arrow exists under the video, but nothing is loaded, and the link does not work, it blocks, without succeeding! For example  : 

https://r3---sn-hgn7rne6.googlevideo.com/videoplayback?c=WEB&ei=k9Z_W8q8LKXGxgK934_gCQ&lmt=1523474186779311&expire=1535126259&ratebypass=yes&sparams=dur%2Cei%2Cid%2Cinitcwndbps%2Cip%2Cipbits%2Citag%2Clmt%2Cmime%2Cmm%2Cmn%2Cms%2Cmv%2Cpl%2Cratebypass%2Crequiressl%2Csource%2Cexpire&itag=22&mn=sn-hgn7rne6%2Csn-4g5e6nzl&ipbits=0&mm=31%2C26&mt=1535104486&pl=2...