HTML was "based on" SGML only in the sense that it borrowed a lot from SGML. However in practice it was never an application of SGML. HTML4 tried its best to mo

HTML was "based on" SGML only in the sense that it borrowed a lot from SGML. However in practice it was never an application of SGML. HTML4 tried its best to move developers to SGML based HTML but devs ignored it. HTML5 recognises that there was this gulf between the specification and the actual usage and sided with real world usage.