Is this just the holder.js stuff or something else as well? I imagine we could replace holder.js with some SVGs, 1x1 GIFs, etc?

Yup, mostly holder.js. I also suggested SVGs in #27395 but I didn't see any traction. Plus Liquid does not do regular expressions so this cannot be extended any more than exact matches...

All this will be useless with Hugo anyway, but since I don't know when I'll get to finish that, you know.

I don't see these many limitations/issues, to be honest. There is one issue, to be precise.

Isn't the real question to ask, what's the main focus of the html markup example? It's primarly showing and teaching the element structure and bootstrap class naming. The src of the image is secondary.

There are some image placeholder services available with a similar url structure as holder.js. Easiest would, replace it by e.g. //imgplaceholder.com instead of ... ✌️

Well we are not tied to any solution. I just need something that works properly and the current solution does not. It's so simple.

Putting this on hold for now. I still don't care about using the plugin or not as long as it works for our needs. That being said, it's a good chance to replace holder.js with pure SVGs.

Removed from v4.2 given the hold.

Edit: Doh, missed the unlock!