JSFiddle - React, Tailwind, and code Playground

CSS

div.test {
    position: fixed;
    left: 0;
    top: 0;
    width: 100mm;
    height: 100mm;
}

div.view {
    border: 1px solid black;
    padding: 0;
    margin: 0;
}

JavaScript

function mm2px() {
    var e = document.createElement("div");
    e.className = "test";
    document.body.appendChild(e);
    var rect = e.getBoundingClientRect();
    document.body.removeChild(e);
    return rect.width / 100;
}

function div(size) {
    var e = document.createElement("div");
    e.className = "view";
    e.textContent = size;
    e.style.width = size;
    e.style.height = size;
    document.body.appendChild(e);
}

var factor = mm2px();
var mm = 72;

div(mm + "mm");
div((mm * factor) + "px");