Question

I am looking to detect with Javascript the fastest possible way, if there is a HTML tag with attribute [itemtype='http://schema.org/Offer'] in the current page.

Answer 1

if(document.querySelectorAll("*[itemtype='http://schema.org/Offer']").length){
    alert("It is there!");
}

Use Sizzle for IE 7 and below. It is the same CSS selector engine for jQuery, but without all those fancy stuff.

document.querySelectorAll = document.querySelectorAll || Sizzle;