NOT lazyload; click a button instead anywhere in the whole <iframe> element so one won't mis-click; will ignore small / narrow <iframe>s.
Per
// ==UserScript==
// @name <iframe> click to load
// @name:zh-CN 手动加载 <iframe>
// @description NOT lazyload; click a button instead anywhere in the whole <iframe> element so one won't mis-click; will ignore small / narrow <iframe>s.
// @description:zh-CN 不是 lazyload；点击按钮而不是点击 <iframe> 元素，以免误触；会忽略大小较小的 <iframe> 元素。
// @namespace RainSlide
// @author RainSlide
// @version 1.0
// @match *://*/*
// @grant none
// @inject-into context
// @run-at document-end
// ==/UserScript==
document.querySelectorAll('iframe').forEach(
iframe =>
// attribute
iframe.getAttribute("src") !== null &&
iframe.getAttribute("srcdoc") === null &&
// content size
iframe.clientWidth >= 72 &&
iframe.clientHeight >= 72 &&
( iframe.clientWidth + iframe.clientHeight ) >= 256 &&
iframe.setAttribute("srcdoc",
`<style>
html, body { height: 100%; }
body {
display: flex;
flex-direction: column;
justify-content: center;
align-items: center;
box-sizing: border-box;
min-width: 5em;
margin: 0;
padding: .5em;
border: 2px dashed currentColor;
}
a {
margin-bottom: 1ex;
word-break: break-all;
font-family: monospace;
}
</style>
<a href="${ iframe.src }" target="_blank" rel="noreferrer noopener">${ iframe.src }</a>
<button onclick="window.frameElement.removeAttribute('srcdoc')">${
new Map([
["en-US", "Load"],
["zh-CN", "加载"],
["zh-TW", "加載"],
["zh-HK", "加載"]
]).get( navigator.language ) || "Load"
}</button>`
)
);