For its UI support probably.
That's also the reason why I work on Pas2JS_Widget: I really like the idea of being able to use Lazarus' GUI designer aka known mechanisms to implement the UI.
I can't imagine to create modern web app UI using designer. Too many html/css/js related things in real life. (yes I'm crazy man, I need CSS in LCL too
). Maybe for very simple projects it will be great (Pas2JS_Widget or TMW WebCore).
Well, I
can imagine it.

Your project is very promissing and I don't get it why it is not No.1 priority for core Lazarus developers to join you in this effort and to integrate it in the Lazarus as soon as possible.
I am afraid that this window of opportunity for Lazarus might be lost.
Well, I'm not the original author. Though it seems that I'm the only one still working on that and right now I'm improving it for my own requirements...
That is the unfortunate of Open Source Projects everyone try to do same thing but there own way instead of join as one team.
Imagine if Lazarus and CodTyphon team join together and created the IDE.
Please no. The CT Team already results in enough headaches.
Same way few programmer try to create past2js widget set there own way right now we don't have any usable project in hand.
The problem is that it's not clear yet what the best approach for web development with pas2js is, the platform is simply too young with too few developers playing with it.