WebGL exposes the graphics card to JavaScript: GLSL shaders, textures, and geometry rendered inside any modern browser at display rate. Since roughly 2011 it has made the browser a legitimate performance surface, with libraries like three.js as the standard toolkit.
For visual software the distribution story is the point: no installer, no platform builds, a URL or local page and the GPU is working. Shadertoy built a worldwide shader culture on it.
Photism’s renderer is WebGL via three.js, which is why its output opens as a browser window: fullscreen it on any display and the browser disappears, leaving the scene.