To convert an HTML document to XHTML, you will need to make sure that the document follows the rules and syntax of XHTML. XHTML is a variant of HTML that is based on XML and has stricter rules for formatting and structuring the document.

Here are some general steps you can follow to convert an HTML document to XHTML:

1. Make sure the document has a valid DOCTYPE declaration. In XHTML, the DOCTYPE declaration must specify the version of XHTML being used and the location of the DTD.

2. Use lowercase tags and attributes. In XHTML, all tags and attributes must be in lowercase.

3. Close all tags. In XHTML, all tags must be closed, either with a closing tag or with a self-closing tag. For example,
   should be written as
   and

   should be written as

   .

4. Use quotes around attribute values. In XHTML, all attribute values must be enclosed in quotes.

5. Use the & character to encode special characters. In XHTML, the & character must be used to encode special characters such as < and &. For example, < should be written as < and & should be written as &.

6. Validate the document. After making these changes, it is a good idea to validate the document to ensure that it is well-formed and follows the rules of XHTML. You can use an online HTML or XHTML validation tool to check the document for errors.

For more information on XHTML and how to convert an HTML document to XHTML, you can refer to the following resources:

• W3Schools: https://www.w3schools.com/html/html_xhtml.asp
• MDN: https://developer.mozilla.org/en-US/docs/Web/XHTML