Possitioning for 3d.js

Am trying to position an element so it will remain below all other as an animated backgroud.

var svg = d3.select("html").append("svg")
    .attr("position","absolute"),
    .attr("z-index","-6"),
    .attr("width", width)
    .attr("height", height);

That did not work.
I also tried to do that in the html by declaring

html{
position:relative;
margin:0px;
width:100%;
height:100%;
z-index:-7
}
svg{
position:absolute;
margin:0px;
top:0;
left:0;
width:100%;
height:100%;
z-index:-6
}

that didn’t work. Anyone could please suggest a solution?
thx
D