I do not see anywhere it saying HTML4 or HTML5, and do not know about HTML enough to realize if it matters.
a web server implements the HTTP protocol, it doesn't need to know anything about CSS, HTML, etc....
the WebServer example of networking libraries shows how to use the TCP Server class EthernetServer or WiFiServer by creating a very simple web server which handles a HTTP request and sends a HTTP response with a very simple HTML page
The receiver (e.g. your browser) is the one that cares about this since it is the one that must parse and render it properly. The server does not care.