Which version of HTML introduced different document types?

HTML 4.01 is the version that introduced different document types through the use of Document Type Definitions (DTDs). This was significant because it allowed developers to specify which version of HTML they were using, thus clarifying how browsers should interpret the markup. With HTML 4.01, the distinctions between strict, transitional, and frameset document types were established.

This differentiation was essential for ensuring consistent rendering across different browsers and served as a way to help developers choose which features they wanted to use based on their needs. The strict version of HTML 4.01 ensured that developers adhered to certain coding standards, while the transitional version allowed for more leniency, accommodating content that might not strictly adhere to the new standards.

In contrast, HTML 2.0 did not provide any formal mechanisms for document types, and HTML 3.2 came before the DTDs established in HTML 4.01. HTML5 did eventually build upon these concepts but focused more on new features and modern practices rather than introducing document type distinctions in the same foundational way.