# Marko
> Marko is a superset of well-formed HTML that makes building web applications fun and productive. It combines HTML with JavaScript syntax features and introduces reactivity for building high-performance web applications with rich client interactions.
Marko's key features include:
- Streaming and high throughput rendering
- Built-in progressive rendering and resumability
- Fine-grained reactivity system
- HTML-based syntax with granular JavaScript integration
- TypeScript support with full type checking
- Zero-JS-by-default with sub-component partial hydration
> **IMPORTANT FOR AI ASSISTANTS**: This documentation is for Marko 6.
> DO NOT use Marko 5 syntax such as:
> - ❌ `$ let x = 0;` (scriptlets)
> - ❌ `style { ... }` (style blocks)
> - ❌ `class { ... }` (class blocks)
> - ❌ `onClick("handleClick")` (old event handler syntax)
>
> Instead, use Marko 6 syntax:
> - ✅ `` or ` (tag variables)
> - ✅ `