Is that "straight-forward easy to understand and follow JavaScript" the whole thing written from scratch? Or does it use libraries (that use libraries, that use libraries)?

Because I've written my share of javascript-from-scratch in my time - before npm and such. And even if my use-case was limited, in order to get edge-cases and details working - issues long solved by their HTML/CSS counterparts - we needed more and more JS. Many of which handwritten polyfills, agent-detection, etc.

Seriously, things like scrollbars (because the client insisted on them being consistent across user-agents) or dropdowns (because they had to be styled) "visited" state on links, in pure JS are thousands of lines of code. Maybe not today, anymore, IDK, with current APIs like the history API or aria labeling. But back then, just in order to make the dropdown work with screen readers, or the scrollbars react well to touchpads -in the direction the user was used to based on their OS- took us thousands of lines of JS, hacks, workarounds and very hard to follow code - because of the way the "solutions" were spread out over the exact right combination of JS, HTML and CSS. Edit: I now recall we got the web-app back with the comment "When I select "Language" and start typing "Fr" I expect French to be picked and "enter" to then put the language in French". We spent another few days on functions that collect character inputs in memory and then match them with values. All because "flags in front of the names were of crucial importance".

So, maybe this is solved in modern HTML/CSS/JS. But I highly doubt it. I think "some straight-forward ... JavaScript" is either an `import { foo } from foobar` or a pipe-dream in the area of "programmers always underestimate hours"