HTMLコーディング規約 (HTML/CSS Style Guide)
Google - https://google.github.io/styleguide/htmlcssguide.html
@mdo - https://codeguide.co/
WordPress (CSS) - https://make.wordpress.org/core/handbook/best-practices/coding-standards/css/
WordPress (HTML) - https://make.wordpress.org/core/handbook/best-practices/coding-standards/html/