Coding standards

• WordPress PHP Coding Standards.
• WordPress HTML Coding Standards.
• WordPress CSS Coding Standards.
• WordPress JavaScript Coding Standards.

Tools for checking standards

PHP_CodeSniffer: WordPress Coding Standards

Documentation standards

• PHP Documentation Standards.
• JavaScript Documentation Standards

Best practices for plugins

For single-file plugins, we can take as a sample wp-comment-notes plugin.

For multi-file plugins there is a great WordPress Plugin Boilerplate.