You can set options that instruct Dreamweaver to automatically clean up your hand-written code according to criteria that you specify. However, Dreamweaver never rewrites your code unless the code rewriting options are enabled or you perform an action that changes the code. For example, Dreamweaver does not alter your white space or change the case of attributes unless you use the Apply Source Formatting command.
A few of these code rewriting options are enabled by default. For information on how to disable them, or enable others, see Setting code rewriting preferences.
The Roundtrip HTML capabilities in Dreamweaver let you move your documents back and forth between a text-based HTML editor and Dreamweaver with little or no effect on the content and structure of the documents original HTML source code. These capabilities include the following:
<MyNewTag><em>text</MyNewTag></em>)
, Dreamweaver marks it as an error but doesnt rewrite the code.