When you create a page, you can make it XHTML-compliant. You can also make an existing HTML document XHTML-compliant.
Open a document, and do one of the following:
For a document without frames, select File > Convert, and then select one of the XHTML document type definitions.
For example, you can make an HTML document XHTML-compliant by selecting XHTML 1.0 Transitional or XHTML 1.0 Strict from the pop‑up menu.
For a document with frames, select a frame and select File > Convert, and then select one of the XHTML document type definitions.
To convert the whole document, repeat this step for every frame and the frameset document.