In practice, I know of no sites offering E-texts in this format. There are a large number of available tools, the simplest being the humble text editor. Few handle the more complex parts of HTML well, although that may well not be a problem for the average E-text, consisting mainly of paragraphs of text. Earlier versions of HTML, prior to the advent of style sheets, were fairly basic in the application of style. But the formatting options remain limited, and cannot be applied globally.
Multilingual support is possible to the extent that fonts are available for the required characters.
There is no way to control printing, even to the extent of specifying page breaks. The use of existing HTML tags and attributes for controlling presentation is now discouraged by the HTML 4.0 standard in favour of style sheet elements. Tools for the creation of SGML documents are not widely available, and tend to be for specialist users only. Creation of new DTDs would seem to be beyond the capacity of non specialists. At time of writing, the only readily available and accessible viewer for the end-user is Panorama, from Interleaf.