For HTML5 serialization, "5.0" is recognized but "5" is not
Saxon recognizes the value "5.0" as the value of the html-version or version attribute when deciding whether to use HTML5 (or XHTML5) serialization. The specification suggests that the value "5" (or indeed, any decimal number equal to 5) should be recognized.
A patch is being committed.
Note that Saxon does not necessarily implement all the details of HTML5 serialization yet.