WASM = https://en.wikipedia.org/wiki/WebAssembly , or does that stand for something else? Asking at that page has zero results for "applet".

How is "safe" defined, and how to find out if something is "safe"?

Is this basically a very specific implementation, trying to solve a very small part of T56213?

WASM = Web assembly.

I use the term applet, in the same sense 'java' applet was used. Is there a better term? One of the intended uses case was 'semi-interactive' items that on another site might use Flash ( although that's now deprecated) , to present a concept or content in a way that text, images or other media cannot.

"Safe" means sandboxed sufficiently strongly that it can't 'leak' code or information outside of the sandbox, or in ways that would arbitrarily effect the rest of the browser or wiki page with nagative consequences. There were some comments on the Discord, that this might not be feasible because of concerns about being able to effectively sandbox any aribtary javascript running in wiki page.

Yes, this was partly about 'data representation', (a small part of T56213) but it was also about having the ability for competent developers, that were willing to have their applet code torn to shreds in public, being able to use a suitable WASM applet ( which need not be the BBC BASIC one mentioned.) , to develop semi-interactive content so support specific pages where some degree of interactivity with the content would aid understanding. Some example use cases being simulations of probability, gravitation in planetary system, engineering dynamic systems, electrical circuit behavior, hydrological systems, biochemistry reactions and so on where although a simple animation or static image can explain things, a fuller simulation would aid understanding further. (Naturally the relevant content article if of a good standard should note the limitations of any simulation.)

These 'applets' could also be written directly in an appropriate tool, and compiled for WASM, they don't have to be interpreted BBC BASIC running in the applet whose source I linked. ( I like BBC BASIC because it's one consistent API for most things. )

There are some examples of the specific BBC BASIC linked from a forum thread here - https://stardot.org.uk/forums/viewtopic.php?f=63&t=20383
(Most aren't relevant to Wikimedia projects)

The actual BBC BASIC for SDL applet is here - https://www.bbcbasic.co.uk/bbcsdl/wasm/bbcsdl.html, examples are *.BBC files in the sub-directories presented by the interface.)

Aside: Amongst the examples, which aren't necessarily open source sadly, was a simple jigsaw puzzle, under Games. This is something that would otherwise be done in Flash (which is now depreciated), and was something I had previously raised a low priority phabricator ticket about, in order to make use of Commons images for something 'fun'.

Has existing interest been expressed somewhere by some WASM folks about Wikimedia, which could be linked to?

I am not aware of any yet., so if this ticket is kept open it should be marked as low priority.

Some more information on WASM here - https://webassembly.org