标准通用标记语言（Standard Generalized Markup Language，SGML）是现时常用的超文本格式的最高层次标准，是可以定义标记语言的元语言，甚至可以定义不必采用< >的常规方式。由于它的复杂，因而难以普及。

这是一个它的例子：

同时它也是一个ISO标准："ISO 8879:1986 Information processing -- Text and office systems -- Standard Generalized Markup Language (SGML)"

The Standard Generalized Markup Language (SGML; ISO 8879:1986) is for defining generalized markup languages for documents. ISO 8879 Annex A.1 defines generalized markup:

HTML was theoretically an example of an SGML-based language until HTML 5, which admits that browsers cannot parse it as SGML (for compatibility reasons) and codifies exactly what they must do instead.