HTML 5 is the fifth major revision of the core language of the World Wide Web. This document outlines the guiding principles that the HTML Working Group used in developing HTML5. The principles focus on compatibility, utility, interoperability, and universal access. The aim is to design HTML in a way that is compatible with existing content, degrades gracefully in older systems, avoids unnecessary complexity, solves real-world problems, supports multiple constituencies, ensures security, separates content and presentation, defines well-defined behavior, handles errors effectively, and promotes universal access. The principles are not absolute, but rather pragmatic rules of thumb. No controversial or surprising information is presented in this content.
https://www.w3.org/TR/html-design-principles/