[KevinH]

all epubs require xhtml files not html files. Xhtml files are true xml with case sensitivity and proper closing tag for every opening tag, etc. html code has much fewer restrictions and can omit closing tags such as a closing p tag in many circumstances leaving the code a bit of a spaghetti hash.

Early versions of epub3 required the .xhtml extension, now it is just recommended. Using .html is allowed but not encouraged. No matter what the extension, the contents are xhtml not html so matching the extension to the contents just makes the most sense.