I usually download some videos from youtube to watch them later, offline. Since Youtube switched to HTML5, all videos are not downloaded as .webm. The problem I have got is that either VLC Player or WebM Free Player seem not able to play those videos on my PC. VLC has CPU spikes, while WebM Player plays them sluggishly. Any other player should I try?
Try Media Player Classic @ http://mpc-hc.org/downloads/ (they also provide non-installer versions) I use youtube-dl ( https://rg3.github.io/youtube-dl/ ) to save youtube videos. It's a command-line program but I find it more versatile than those extensions or greasemonkey scripts.