Current browsers correctly interpret omissions in HTML instructions. In contrast, XHTML code must be well formed by adhering closely to established standards. To give an example, if there is a missing </table> end tag, Internet Explorer will fill it in. In XHTML, this would be an example of ill-formed code and it would probably
result in a failure to show the table.

There are a number of considerations in writing well-formed XHTML code. Keep in mind that some of these provisions are a stricter enforcement of things that should have been done in HTML 4, but you could get away with errors and the browser would correct many of them. This is no longer true in XHTML.

Required tags
In XHTML, the head and body tags must be included. Since including them is part of well written HTML code anyway, this shouldn’t be a problem.

Tags must be properly nested
Tags must be closed in the proper order. In HTML, if you were using the italic tags in a paragraph, it didn’t matter which tag you closed first, and which tag was closed second. In XHTML, tags need to be closed in reverse order—the last tag opened is the first closed.