可以通过外部资源调用，亦可以直接在<body></body>标签里使用<script></script>来实现
(function () {
var encodedValidDomain = 'd3d3LnRlMjYwLmNvbQ==';//base64加密后的域名
var encodedredirectUrl = 'aHR0cHM6Ly93d3cudGUyNjAuY29t'//base64加密后的链接，包含协议头
var decodedValidDomain = atob(encodedValidDomain);
var redirectUrl = atob(encodedredirectUrl);
var hostname = document.location.hostname;
function createWatermark(text) {
var watermarkDiv = document.createElement('div');
watermarkDiv.style.pointerEvents = 'none';
watermarkDiv.style.position = 'fixed';
watermarkDiv.style.top = '0';
watermarkDiv.style.left = '0';
watermarkDiv.style.width = '100%';
watermarkDiv.style.height = '100%';
watermarkDiv.style.zIndex = '9999';
watermarkDiv.style.opacity = '0.1';
watermarkDiv.style.background = 'transparent';
watermarkDiv.style.overflow = 'hidden';
watermarkDiv.style.display = 'flex';
watermarkDiv.style.justifyContent = 'center';
watermarkDiv.style.alignItems = 'center';
watermarkDiv.style.flexWrap = 'wrap';
var watermarkText = document.createElement('div');
watermarkText.innerText = text;
watermarkText.style.color = 'black';
watermarkText.style.fontSize = '30px';
watermarkText.style.transform = 'rotate(-30deg)';
watermarkText.style.whiteSpace = 'nowrap';
watermarkText.style.margin = '20px';
for (var i = 0; i < 100; i++) {
watermarkDiv.appendChild(watermarkText.cloneNode(true));
}
document.body.appendChild(watermarkDiv);
}
if (hostname !== decodedValidDomain) {
createWatermark(decodedValidDomain);//decodedValidDomain可以替换为自定义水印内容
var userResponse = confirm("警告：您当前浏览的页面非官方页面，可能存在有害信息！建议您跳转至官方页面进行浏览！");
if (userResponse) {
window.location.replace(redirectUrl);
}
}
})(); !
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