Nowadays, validation and consistency of Html files is a hot subject. Most browsers accept a wide range of either valid, according to previous standards, or invalid structures. However, awareness on the HTML5 is an important concern, in spite of the extensive effort required to check assets.
In the evolution of the Internet, several Html standards has been implemented and are still very popular. As there is no way to purge the whole network, they are going to be around forever. In this domain, the same situation is found, a great deal of work is required to certify and maintain compliance with a standard.
From a historical point of view, plain index.html were introduced to avoid directory browsing. At these times, they are not longer necessary as directory browsing prevention is a common security measure. In this line, JED Joomla! Extensions Directory (JED) has recently dropped the requirement for index.html in submitted extensions.
The benefit of HTML5 is clear. It's implemented to ease the transition to modern devices and support new technologies. Thus, some experts support a mandate for pure HTML5 pages. In small projects or controlled environments, it's likely to be achieved and the benefits can be verified.
In Joomla! community project, anyone can promote a full compatibility effort. Github is easy to use, and a specific editor is not even necessary. As a solution, all index.html can be dropped or modified to implement the above detailed structure. Only precious time for the task is required.