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XHTML stands for Extensible Hypertext Markup Language, and is very similar to HTML. Both are designed to allow you to create electronic documents that are commonly formatted for web browsers such as Internet Explorer and Firefox. The most important difference is that XHTML requires consistency and structure in the code.
When starting a new document, you may decide to make it XHTML compliant.
From the File menu, select New...
The New Document dialog box appears.
Select the desired Page Type and Layout
From the DocType pull-down list, select an appropriate XHTML option
NOTE: The DocType pull-down list is not available for Other and CSS Style Sheets pages.

Click CREATE
The new document is now XHTML compliant.
You may want every new document you create to be XHTML compliant. To do this, you can create XHTML compliant documents by default.
From the Edit menu, select Preferences...
The Preferences dialog box appears.

From the Category menu, select New Document
From the Default Document Type (DTD) pull-down list, select the appropriate XHTML document type definition
Click OK
Any new documents will be XHTML compliant.
After creating a document, you may choose to make the document XHTML compliant.
NOTES:
When working with frame pages, you must convert each frame and the frame set document.
If a template-based web page is based from a non-XHTML template, than it cannot be converted to a XHTML web page. Template-based web pages always need to remain in the same language as the original template.
Open the document you wish to make XHTML compliant
From the File menu, select Convert » appropriate XHTML selection
The document is now XHTML compliant.