browser mode switches on the Developer tool send the UAS for that version.
alternatively you can use F12> Tools>Change User Agent
this is for legacy markup that is using UserAgent Sniffing...
document Mode... there is Quirks, Near Standards (IE7 hasLayout) and Standards
differnet versions of IE have different levels of support for Standards.... document mode and the x-ua meta is for backwards compatibility.
simple solution ... use the html5 or system DTD so that all browsers and browser versions use the most Standards compliant rendering that they are capable of. Use Feature testing instead of UAS sniffing.