Four camps

Camp	Examples	What it costs
WebView	Electron, Tauri, wails, Neutralino, Photino	The UI is HTML rendered by a browser engine. Behaviour follows whichever WebKit the machine has.
Own renderer	Flutter, GPUI, Sciter, Kivy	The same everywhere, and nothing in it is native.
Widgets via a runtime	NodeGui, Proton Native, Toga, PyQt, Fyne	Real widgets, but Node/Python/Qt/GTK ship with the app.
System script host	WelsonJS, HTA	Nothing bundled — and Windows only, with MSHTML doing the rendering.

Spark is in none of them: HTML-ish markup, real OS widgets, the system's own JavaScript engine, all three desktops. In a survey of some 180 alternatives, no project hit all four.

What Spark gives up

• The npm UI ecosystem. No component library, no CSS framework, no React. The vocabulary is fixed and small.
• Windows' engine is old. Chakra is ES5.1. The bundling step makes npm code usable, but that ceiling is real.
• Linux accessibility. AT-SPI is D-Bus, and Spark carries neither it nor a toolkit that speaks it. macOS and Windows get it for free.
• Adding a widget Spark does not map yet is C, in three hosts.

What it is unusually good at

• Size and idle cost. One file of a few hundred kilobytes, no Chromium in memory, no render loop when nothing moves.
• Measured sameness across three native toolkits. Everyone else with real widgets accepts that each platform looks different; here it is checked on every change, in pixels.
• Accessibility for free on the two systems that offer it, because the controls are real.
• Nothing to keep patched. No bundled engine means no bundled engine's security updates.