I've seen this question over and over , to make the HTML5 code working in SharePoint 2010 simple modifications to the OOTB master page is needed

  1. replace the first line <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">   with <!DOCTYPE HTML">
  2. Change the compitability to IE=9 instead of 8  by replacing <meta http-equiv="X-UA-Compatible" content="IE=8"/>   with <meta http-equiv="X-UA-Compatible" content="IE=9"/>