So, what is a DSL? A DSL is a computer language targeting a specific problem or domain space. This is in contrast to general-purpose programming languages that are meant to be capable of solving general computing problems. HTML is an example of a domain-specific programming language as it's meant to be used for a very specific problem.
The distinction between general-purpose programming languages and domain-specific languages can be fuzzy at times. For example, HTML has a very clear and focused purpose, whereas Kotlin can be used for general computing problems as well as for building custom DSLs for specific problems.