The web server that serves Zen web pages
The web server that serves Zen web pages could do such things as spawn a headless web browser with the Selenium web driver embedded in it. This web driver could be controlled by the web server to do such things as copy a web page and rebase URLs in the web page, so that relative paths are not broken.
A web browser extension that allows web page embedding
The <iframe> HTML tag allows a web page to be embedded in another web page, but due to the clickjacking problem, most web servers send X-Frame-Options HTTP response headers with the web pages they serve to prevent the embedding of those web pages in other web pages. Google Chrome (and probably other web browsers) has an extension to ignore these response headers, thus allowing any web page to be embedded. This extension should be used cautiously to avoid clickjacking attacks. This 100-second video shows how to install and use the Chrome extension.
Some JavaScript tricks
Here’s a trick to take a snapshot of part of a web page and here’s a trick to take a snapshot of a full web page. Note that the snapshots cannot include a web page embedded in an <iframe> element.
A feature in Chrome 90 to create links to highlighted text in a web page
A short article on The Verge describes this new, interesting feature of Google Chrome.