Using Open graph meta property

I've been trying to find a straight answer to this for over 3 hours now.

I want to have a facebook-share on my site. And to to that I need <meta property> tags. However this is not supported without some special import as I understand it.

What I want:

<meta property="og:url" content="http://ift.tt/1k8hA0U" /> 
<meta property="og:title" content="Chocolate Pecan Pie" />
<meta property="og:description" content="This pie is delicious!" /> 
<meta property="og:image" content="http://ift.tt/1FOgNJk" /> 

However this gives the error "Validation (XHTML5) attribute 'property' is not a valid attribute of element meta".

Ive tried changing doctype like in this answer but that did nothing. Ive tried using prefix but that doesn't work either. What am I missing here? I really wish facebook would have a better tutorial for beginners.