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I find this website quite useful: https://www.youloop.io/ when there is no soundslice.com for a youtube video.
"Create loops. Slow down youtube videos. Easily! Create a music lesson from any YouTube video"
You should be able to put any YouTube video into the soundslice, I do it all the time.
I could never find a way of doing bookmarks in soundslice.com. Is there one if I just put the video into soundslice without any notation? I like the ability that youloop has of defining say "intro", "solo", "outro" loops on backing tracks.
I've never looked for bookmark feature, never really had a need for it but I can see it being handy. I'll check out the youloop.