You should try this...
#roll{
width:500px;
margin-right:auto;
margin-left:auto;
}
p{
position:relative;
font-size:1.5em;
z-index: 1;
}
.codepoem{
width:500px;
height:200px;
}
.codepoem iframe{
position:relative;
width:calc(500px + 20vw);
margin-left:-10vw; /* center horizontal */
height:calc(200px + 40vh);
margin-top:-20vh; /* center vertical */
/*z-index:1;*/
}
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