Markdown
- Markdown – Markdown documentation by John Gruber
PHP
- PHP Manual – A PHP language reference
- Composer – A dependency manager for PHP
- phpDocumentor – A tool that makes it possible to generate documentation directly from your PHP source code
WordPress
- Code Reference – An information about WordPress API, functions, classes, methods, and hooks.
- Twenty Twenty Theme – Twenty Twenty is included in Core as of WordPress 5.3