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I suspect it's not a problem for YouTube, and I doubt Google would penalize a site too much for doing so. That said, in general it would be considered best practice to keep your text visible. If you do need to hide text or include it in popups or slideouts for the sake of usability, that's fine too as long as it's not done to excess. In cases like these I think its best to keep an open mind and experiment if necessary, but I wouldn't expect to see a big difference either way.