Be sure to set the DOCTYPE prior to creating any web pages.
1. Click Tools
2. Choose Page Editor Options
3. A Dialog window with several Tabs opens. Choose the Authoring Tab.
4. Scroll down to the middle area where it reads, "DOCTYPE and Secondary Schema".
5. Choose a Doctype - Choose HTML5 for the Doctype and Secondary Schema
6. Press OK.
That's all there is to it. You only have to do this one time for your web site.
The doctype is used to tell the web browser the particular version of html used to create the web page. It's also responsible for triggering a particular browser rendering mode.
Two general browser rendering modes exist.
(In reality, some browsers have 4 or 5 different browser rendering modes. Firefox, for instance, has several modes. I am simplifying this discussion merely to make a point.)
Ideally, we will choose a Doctype that will trigger Standards Mode. As a result, our web pages will be rendered according to the way we specified CSS Styles etc, which is based upon the current Web Standards.
Simply specify a valid doctype and standards mode will be triggered. Use an outdated doctype, or misspell a doctype and you will trigger quirks mode. Omitting the doctype will also trigger quirks mode.
Quirks mode renders web pages according to older browsers which were not based upon current web standards. (you can read about quirks mode at: http://www.quirksmode.org/css/quirksmode.html)
In a short while, HTML5 will be the new web standard. Acutally, it's slated for 2012. So it makes sense to start using it now.
Backwards compatible: the HTML5 Doctype is backwards compatible. So to become compliant with the new web standard, all you need to do is use the HTML5 doctype. That's it. It would be nice if you can change over to HTML5 all together. But you don't have to at this time.
That purpose is to trigger Standards Mode!!