Learn HTML 4. Build some applications with it and then learn HTML5. It isnt a big learning curve but the most important part is to actually play around and TRULY understand how HTML4 works. Then when you get to HTML5 you will learn what changed and you will understand why. HTML 5 is still unofficial. If I recall correctly, it is officially out in 2014. But you can still use the HTML5 doctype and other features in your projects, they just wont be compatible across different browsers.
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Learn HTML 4. Build some applications with it and then learn HTML5. It isnt a big learning curve but the most important part is to actually play around and TRULY understand how HTML4 works. Then when you get to HTML5 you will learn what changed and you will understand why. HTML 5 is still unofficial. If I recall correctly, it is officially out in 2014. But you can still use the HTML5 doctype and other features in your projects, they just wont be compatible across different browsers.