HTML and XHTML are two markup languages used to create webpages. Both languages use tags and attributes to structure content, but there are some crucial differences in the syntax and rules between the two. One significant difference between HTML and XHTML is their adherence to syntax rules. HTML has loose syntax rules, meaning that the tags and attributes are often not closed or formatted correctly. In contrast, XHTML is stricter with syntax rules and requires all tags and attributes to be closed and formatted correctly.To convert an HTML document to XHTML, follow these steps:1. Change the document's DOCTYPE declaration to the XHTML format:

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```2. Close all the tags that are not closed and format all attributes properly.3. Add a namespace attribute to the root HTML element:

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```4. Change all tags to lowercase.5. Make sure that all tags have their corresponding closing tag.By converting an HTML document to XHTML, it becomes possible to use extensible features, such as adding custom elements and attributes, and the document can be processed by XML tools.In conclusion, HTML and XHTML have similar goals, but their syntax rules and usage differ. Converting an HTML document to XHTML requires adhering to stricter syntax rules and changing the document's DOCTYPE declaration. Citation:- For more information about XHTML, please visit https://www.w3schools.com/html/html_xhtml.asp- To learn more about the differences between HTML and XHTML, please visit https://www.htmlgoodies.com/html5/tutorials/the-difference-between-html-and-xhtml.html