One of the things that I always find missing in these libraries/collections is a date/time picker. I know these are available via many separate projects, but it's nice if one picks a collection like that, that all inputs are homogeneous.
That's only an "issue" of all that "modern" stuff. Many, many years ago I've used Dojo[1] and it's component lib Dijit. It had all that complex widgets in the toolbox. With all the "advantage" stuff like ARIA and internalization. Or you could use Ext, if you could pay it. But than all this "modern 'component' stuff" came form nowhere and you've been back to stone age, where you didn't have even the most basic compone…
Still, facts are facts, no matter how sloppy the wording I've chose.
I didn't mean to diminish the effort put into that one incarnation of those so called design systems. I've only mentioned that it's (like all others of the "modern" ones I've seen until now) light years behind what you could have got 10+ years ago.
If I would like to really offend some people here I would have said that actually Visual Basic 6 was far more advanced than today's "web component libraries" when it comes to building GUIs. Most likely this could be even proved: One would only need to build (from scratch) a typical GUI for some business app with web-tech and VB6 against a timer. I know on which tech I would bet delivering results faster. ;-) Not that I'm saying a VB6 app would be a proper replacement for a web app, that's not the point. But the productivity of web tech for application development is just somewhere on the level of 20 years ago. But the down-voters are likely just to young to know that. So the down-votes are likely fine. :-)