JSFiddle - React, Tailwind, and code Playground

JavaScript

window.init1 = function(iDoc, iWin) {
    iDoc.write("<html><head></head><body><div id='testDiv' style='width:1px;padding-left:1px'></div></body></html>");
    $('body').append("<p>offsetWidth of div in iframe1 = "+$('#testDiv',iDoc)[0].offsetWidth+"</p>");
};

window.init2 = function(iDoc, iWin) {
    iDoc.write('<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">'+
               "<html><head></head><body><div id='testDiv' style='width:1px;padding-left:1px'></div></body></html>");
    $('body').append("<p>offsetWidth of div in iframe2 = "+$('#testDiv',iDoc)[0].offsetWidth+"</p>");
};

$('body').append("<iframe src='javascript:parent.init1(document,window);'>");
$('body').append("<iframe src='javascript:parent.init2(document,window);'>");

$('body').append("<p>parent frame boxModel = "+$.support.boxModel+"</p>");