JSFiddle - React, Tailwind, and code Playground

HTML

<!DOCTYPE html>
<html><body>
        <div id="wrap" style="">
        <div id="test">TEST</div>
    </div>
</body></html>

CSS

#wrap {
    width: 400px; height: 400px;
}

#test {
    width: 100px; margin: 10%; background:gray;
}
#test.fail { background:red; }
#test.pass { background:green; }

JavaScript

(function($) {
    var ow=$('#test').outerWidth(),
        owm=$('#test').outerWidth(true),
        
        expected={
            ow : 100,
            owm : 100 + (2*0.1*400)
        };

    if(expected.ow===ow && expected.owm===owm) {
        $('#test').addClass('pass');
    } else {
        $('#test').addClass('fail');
    }
    
    $('#test').html(ow+"/"+owm);
})(jQuery);