Ship SEO-first interactivity without rewriting your HTML.

VSN.js layers behavior onto your server-rendered pages. Keep the HTML your backend already serves, then sprinkle in state, events, and bindings with a CFS layer that feels like CSS.

Read the docs View examples

Install

npm i vsn

Quick behavior

            
behavior .card {

  count: 0;

  on click() { count++; }

}

Why VSN.js

SSR-first by design

Keep markup on the server and enhance it with a logic cascade that respects selector specificity.

CSS-like logic

Behaviors are scoped by selectors, making it easy to reason about state and UI contracts.

Small runtime

Minimal client code, no heavy templating engine, and clean progressive enhancement.

Notes from the creator

We know how much developers love CSS, so building CFS to program website functionality akin to CSS was a no-brainer!

Start here

Get Started

Install, mount, and write your first behavior in minutes.

Docs overview

Examples

Explore tabs, todos, and kanban patterns built with CFS.

Open examples