It works with HTML, but you can also use it to generate templates of any language, like Markdown.
Squirrelly is about 2KB gzipped, so it shouldn't affect your code size much. The runtime is even smaller: 953 Bytes gzipped!
Squirrelly has a number of features that set it apart from the competition:
It's faster than most templating engines
It supports user-defined helpers and native helpers
It supports filters
It supports partials
It's incredibly lightweight: in comparison: the full version only weighs about 2 KB gzipped, compared to Pug's 237 KB and Handlebars' 21.5 KB
It works with other languages than HTML
It's not whitespace sensitive
For more information, see Why Squirrelly?