First to prevent HolyWars let me tell you what JSVim isn't.

• It is not the excellent packaging of vim made by Lu Wang - wish it were
• It is not an Atom-like or NeoVim all-in-one
• It is not an "A few keys implemented" vi-clone, been there with JSVim3.0 - you can't sync vim selection with Range properly in a TextArea
• It is not the quite minimal code spread since decades

That said

JSVim, in its 4.0 version is an HTML5 CustomElement TextArea extension which aims at

• Providing the full xvi runtime through Emscripten - merely done
• Provide a TERM implementation in CustomElement compatible with HTML5 capabilities - fighting for it
• Stay Small ! - NonArguableMatter