Write or paste Markdown? and see it rendered live, side by side. Supports headings, tables, fenced code blocks?, task lists, and all standard GFM? syntax. Copy the HTML? or download — nothing leaves your browser.
Plenty of apps — note tools, wikis, chat, and code sites — let you format text with simple symbols: a # for a heading, stars for bold. Paste or type that kind of text on the left and this page shows you exactly how it will look, updating as you type.
It's an easy way to check a document reads correctly before you post it. Nothing you write here leaves your computer.