Several solutions for mobile adaptation (three solutions)

(function (doc, win) {
var docEl = doc.documentElement,
resizeEvt = 'orientationchange' in window ? 'orientationchange' : 'resize',
recalc = function () {
var clientWidth = docEl.clientWidth;
if (!clientWidth) return;
docEl.style.fontSize = 100 * (clientWidth / 750) + 'px';
};
if (!doc.addEventListener) return;
win.addEventListener(resizeEvt, recalc, false);
doc.addEventListener('DOMContentLoaded', recalc, false);
})(document, window);

(function (win) {
function setUnitA() {
document.documentElement.style.fontSize = document.documentElement.clientWidth / 10 + "px";
}
var h = null;
window.addEventListener("resize", function () { clearTimeout(h); h = setTimeout(setUnitA, 300); }, false);
setUnitA();
})(window);

html {
font-size: 20px;
}
@media only screen and (min-width: 401px) {
html {
font-size: 25px !important;
}
}
@media only screen and (min-width: 428px) {
html {
font-size: 26.75px !important;
}
}
@media only screen and (min-width: 481px) {
html {
font-size: 30px !important;
}
}
@media only screen and (min-width: 569px) {
html {
font-size: 35px !important;
}
}
@media only screen and (min-width: 641px) {
html {
font-size: 40px !important;
}
}
@unit: 40rem;