How to migrate from HTML to XHTML and How to migrate from XHTML to HTML?

As recommended by W3C following steps can be followed for migration of HTML to XHTML (XHTML 1.0 documents):

Include xml:lang and lang attributes on elements assigning language.
Use empty-element syntax on elements specified as empty in HTML.
Include an extra space in empty-element tags:
Include close tags for elements that can have content but are empty:
Do not include XML declaration.
Carefully following W3C’s guidelines on compatibility, a user agent (web browser) should be able to interpret documents with equal ease as HTML or XHTML.
To understand the subtle differences between HTML and XHTML, consider the transformation of a valid and well-formed XHTML 1.0 document into a valid HTML 4.01 document. To make this translation requires the following steps:

The language for an element should be specified with a lang attribute rather than the XHTML xml:lang attribute. XHTML uses XML’s built in language-defining functionality attribute.
Remove the XML namespace (xmlns=URI). HTML has no facilities for namespaces.
Change the document type declaration from XHTML 1.0 to HTML 4.01.
If present, remove the XML declaration. (Typically this is: <?xml version="1.0" encoding="utf-8"?>).
Ensure that the document’s MIME type is set to text/html. For both HTML and XHTML, this comes from the HTTP Content-Type header sent by the server.
Change the XML empty-element syntax to an HTML style empty element (