https://hostednowhere.com/ actually contains a webapp that allows you to build such URLs for a handful of site templates

Yes, it's not communicated very clearly.

data:text/html,%3C%68%31%3E%64%61%74%61%20%55%52%4C%20%69%73%20%6E%6F%74%20%77%65%62%20%73%63%61%6C%65%3C%2F%68%31%3E

Digital restaurant menus!

I like these. I have one which gives you a little HTML playground. It's in my favourites and I use it quite a lot for jotting things down.

data:text/html,<pre onkeyup="(function(d,t){d[t]('iframe')[0].contentDocument.body.innerHTML = d[t]('pre')[0].textContent;})(document,'getElementsByTagName')" style="width:100%;height:48%;white-space:pre-wrap;overflow:auto;padding:2px" contenteditable></pre><iframe style="width:100%;height:48%">

I have one bookmark that will copy to clipboard the current page URL + an anchor to the highlighted text:

    javascript:(function(){navigator.clipboard.writeText(`${window.location}#:~:text=${escape(window.getSelection().toString())}`);%20console.log("copied!");%20})();

Pretty useful to share a specific part of a page to someone else.

For example it will give you this: https://news.ycombinator.com/item?id=47888337#47888930#:~:te...

This also is quite handy for inlining SVGs in CSS, although I believe you have to mark the encoding as utf-8.