As you’ve already seen, creating HTML frames means adding some elements to your repertoire, including the <frameset> and <frames /> elements. Before you do that, however, you’ll need to specify a new DTD and set up your Web document accordingly.
To work with framesets and be XHTML-compliant, you’ll need to use the XHTML Frameset DTD, which enables you to create frames while using the transitional set of elements and attributes. The frameset DTD is added in the same way that the strict or transitional DTDs are added to your page, using a short entry at the top prior to the <html> open tag: