If you use the editor only in HTML mode (or click save while in HTML mode) and insert tags with uppercase letters, they are not correctly transformed to lower case resulting in invalid Xhtml - it is now allowed to have tags with uppercase letters in Xhtml standard.

You can test this on the live site:
Go to: http://cutesoft.net/example/xmloutput.aspx click HTML tab and edit the first image open tag to be upper case. Click submit. Bam the tag is in upper case. Note that the property ConvertHTMLTagstoLowercase is not set to false and the default is true.

---

http://blogs.vizibility.net/sdobrev