Just wanted to add to some clients it also might mean a web page "devoid of any formatting codes, line feeds or encoded or special control characters of any kind", which is a text file. 😉
In some cases, clients mean they want an extensible format for easier universal import/export, i.e. XHTML and there are tools out there such as HTML Tidy which converts HTML to XML. "This conversion is useful for webmasters who are migrating to XML. It can also help XML converts who have to interface with legacy HTML tools." as noted in the web site where you can download HTML Tidy: