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www.w3schools.com www.w3schools.com/html/html5_intro.asp
Check lynda.com, pluralsight.com and w3schools.com
w3schools are the best, belive me
For just HTML5, I would suggest what the other commenters suggested. But for an online resource for almost anything related to programming, or computer science in general, I would suggest http://www.learnr.pro/
I often learn through ebooks, and this link has lots of good ones, especially those related to computer science.
They do have courses too, and videos. But I never tried those.
Have to mention that it's not free, but it has a credit point system. Where each download will cost you some points (very few), and you can earn points.
And the surprising thing, is that you can buy points using your mobile credit through SMS.
Start here (free)Go here (free)http://learn.shayhowe.com/After these two free spots you will have some awesome knowledge where you can filter all other of the HTML5 learning products out there, but start here, and enjoy the journey