WHATWG now consider HTML a living standard now. One reason is because no browser has perfectly matched any HTML version. In fact each browser implements "HTML5" features at its own pace. For example, <canvas> dates back to 2005, before the HTML5 term was commonly used. Even IE8 supports some features considered part of "HTML5", such as localStorage. Thus, whether a browser supports "HTML5" is almost meaningless, nor whether a page uses "HTML5". Individual features matter.