Use xhtml code, otherwise it will rain messages like these. Xhtml is not very different from "usual" html. The main difference is that xhtml HAS to be valid (img tags ending with />, for example
< img src="images/Ebook.png" width="549" height="426" alt="describe the image here" />
and not
< img src="images/Ebook.png" width="549" height="426" alt="" border="0">
and use < br />
and not < br > , otherwise the browser or the validator won't accept it. The slightest mistake and you will get an error message while validating. It may not make the ebook reader crash, but the companies out there selling the books on your behalf prefer valid ebooks, which is why you ought to try to keep your book valid. Your browser is another good validator, at least for the xhtml part: