Through the various examples that you've seen, you may have noticed the use of XHTML for markup. At this time, this is the latest approved and widely used W3C standard, with HTML5 right around the corner. By default, Umbraco is configured to validate and recognize XHTML 1.0 Strict with the option to turn this off. This means that the Rich Text Editor data type, which uses Tidy for HTML cleanup, validates against that W3C specification. You can turn off this validation in the Umbraco settings, but for best practices it is recommended that you leave it intact and write standards-based HTML and Cascading Style Sheets (CSS).
You'll be glad to know that Umbraco is one of the few CMSs that does not alter your markup in any way at runtime. No extraneous tags or comments are inserted into the view; no automatic stripping of tags occurs, or other modifications of your code. There are two exception to this rule (but no, it's not a catch, I promise):
If you prefer to use another editor, Umbraco allows you to do just that by rolling your own Data Type. That is one of the strong sides of the system, it's API.
<xsl:output method=“html” omit-xml-declaration=“yes”/>