Links

“Can I use” provides browser support tables for front-end web technologies on desktop and mobile web browsers.
Can I use

“Can I use” for email clients.
Can I email

Tools

Web documents markup validation
https://validator.w3.org

JSON

JSON tools on MDN

JSFiddle

My examples

Documentation

Linux

Linux

Scripting

GNU Awk user guide
sed, a stream editor

WooCommerce

WooCommerce Documentation
WooCommerce Action and Filter Hook Reference

Commercial

https://wpsandbox.io/