There is sth i dont understand about using em


<div style="width:50%;margin-top:5em;font-size:1.1em>
Something Something Something Something Something 
<br />
Something Something Something Something Something Something
</div>

I have tooked the screenshots of the page that the above code produces at different resolutions. And the final result was although i used em on fontsize the fontsize looks same on every resolutions. But if em is relational why does it same? Or what I need to do to see it differ in different resolutions?