The Doctype is the way we specify the version of HTML being used to create the page. Intellisense will judge the code against the Doctype in order to determine errors and incompatibility warnings. Likewise, a valadator will do the same thing.
[see figure 1]
[see figure 2]
Figure 1: Tools Menu
Figure 2: Authoring Tab
[click on image to see larger image]
*You need to set the Doctype Prior to Creating a Web Page.
*You only need to set the Doctype once per Web Site, unless you need certain pages to have a different Doctype. To avoid changing the Doctype, use a Transistional Doctype as it permits a broader range of HTML tags to be used. (and they will validate)
*If you forget to set the Doctype but haven't created any web pages just yet, you have a second chance to set the Doctype (DTD) when you create your first web page.