Firstly, if there is one piece of software that I want to be fast, lightweight, and most importantly secure, it's my terminal emulator. The hard part isn't drawing characters to the screen (which is what HTML and CSS would help with), it's the VT100 parsing and logic, which can be written in...well, anything, really.
I would first like to point out that the HyperTerm OSX zip is 43mb, and the unzipped HyperTerm.app is 123mb.
Supporting exactly one OS: again, the OS specific bits aren't the hard part. Blitting characters to the screen is easy, and platform independence certainly isn't worth 123mb of overhead.
250k+ Node modules: The number of packages in a package repository is a poor metric for determining anything except the number of packages in the package repository. npm has tons of redundant or ill-maintained packages, and that's even before we discuss whether having a bunch of available packages makes a terminal better at all. In my opinion, it doesn't, and this circles back around to the fact that a terminal needs to be secure. I sure as hell don't want random unaudited code running in the same application that I use to administer systems.
Standardised plugins: what standardised plugins? npm modules? Not sure what you mean here.
Can't open web pages locally: Why should a terminal do that? As a party trick? I don't want to stand in the way of improvement, here, but I also have no problem opening a separate web browser to browse the web.
Javascript hate: It's not hate when people bring up salient points. Just because the web platform is open and everybody is using it right now doesn't mean that it's perfect or above criticism.