function render(){
var inp = document.getElementById("box");
var data = `
<svg xmlns="http://www.w3.org/2000/svg" width="${inp.offsetWidth}" height="${inp.offsetHeight}">
<foreignObject width="100%" height="100%">
<div xmlns="http://www.w3.org/1999/xhtml"
style="font-family:monospace;font-style: normal; font-variant: normal; font-size:13.3px;padding:2px;;">
${inp.value} <i style="color:red">cant touch this</i>
</div>
</foreignObject>
</svg>`;
var blob = new Blob( [data], {type:'image/svg+xml'} );
var url=URL.createObjectURL(blob);
inp.style.backgroundImage="url("+URL.createObjectURL(blob)+")";
}
onload=function(){
render();
ro = new ResizeObserver(render);
ro.observe(document.getElementById("box"));
}
#box{
color:transparent;
caret-color: black;
font-style: normal;/*must be same as in the svg for caret to align*/
font-variant: normal;
font-size:13.3px;
padding:2px;
font-family:monospace;
}
<textarea id="box" oninput="render()">you can edit me!</textarea>