| ext | Validate (or report) file extensions |
| frontmatter | Validate frontmatter |
| html | Validate HTML files |
| ico | Validate icons |
| jpeg | Validate JPEG images |
| json | Validate JSON files |
| mimetype | Validate (or report) MIME content types |
| png | Validate png images |
| svg | Validate SVG images |
| text | Validate plain text files |
| version | Prints fflint version information |
| xml | Validate XML files |
| yaml | Validate YAML files |