misskey/src/web/app/client/script.js

const head = document.getElementsByTagName('head')[0];
const ua = navigator.userAgent.toLowerCase();
const isMobile = /mobile|iphone|ipad|android/.test(ua);

isMobile ? mountMobile() : mountDesktop();

function mountDesktop() {
	const script = document.createElement('script');
	script.setAttribute('src', '/_/resources/desktop/script.js');
	script.setAttribute('async', 'true');
	script.setAttribute('defer', 'true');
	head.appendChild(script);
}

function mountMobile() {
	const meta = document.createElement('meta');
	meta.setAttribute('name', 'viewport');
	meta.setAttribute('content', 'width=device-width, initial-scale=1, minimum-scale=1, maximum-scale=1, user-scalable=no');
	head.appendChild(meta);

	const script = document.createElement('script');
	script.setAttribute('src', '/_/resources/mobile/script.js');
	script.setAttribute('async', 'true');
	script.setAttribute('defer', 'true');
	head.appendChild(script);
}