replit - Code, create, andlearn together. Use our free, collaborative, in-browser IDE to code in 50+ languages — without spending a second on setup.
ViewHTTPHeaders.com - View HTTP header response for any web page.
JSFiddle - Test your JavaScript, CSS, HTML or CoffeeScript online with JSFiddle code editor.
Sky Live HTTP Headers - Sky Live HTTP Headers is an open-source browser extension that visually displays HTTP requests and response headers from every website you visit.
CodePen - A front end web development playground.
Live HTTP Headers - The goal of this project is to adds information about the HTTP headers in three ways: