Pug is a terse templating/pre-processing language used to build HTML. All the standard practices for HTML apply where relevant to Pug templates as well.
Adapters are available for PHP, Ruby, and of course JavaScript. This means that if suited to a project, a consistent template syntax can be deployed across front and back ends. An adapter does exist for .NET but it's a seemingly unmaintained project.
Pug syntax is familiar if you use Emmet and is influenced by HAML. It is a clean, whitespace-sensitive syntax for writing HTML.
Chain classes directly after an element
input.field.field--large(placeholder = 'Enter your name')