This question already has an answer here:
The WMD (or possibly Prettify) does not support the dot (.) character for elements, despite it being part of the XML Specification. This comes up on StackOverflow with XAML related questions.
Below is an example of the rendering of an element and attribute, both with a "." in their name. Notice that ".suffix" is coloured incorrectly.
(As a new user on this site I cannot mark this question as 'xml' or 'xaml', as was my intention)