YAML ain't markup language. YAML is a concise data serialization language, which is typically considered easier to read than the more verbose JSON or XML. It is most often used in configuration files of servers and software applications. Clark Evans proposed the spec in 2001 as the result of an effort to simplify XML. According to his resume, at the time he conceived of YAML he was working with Python, which likely influenced the syntax of YAML as an indentation-based language.
YAML Checker provides a quick and easy way to validate YAML. As you type, your YAML will be validated with beautiful syntax highlighting and error information.