Read original post by Kirk Schloegel and Brian Hilstrom at BlendInsider
Managing the complexity of interwoven HTML and JavaScript can be a demanding task for designers. One way Blend supports designers is by providing a visual indication in the CSS Properties and HTML Attributes panes when JavaScript values have been set or have overwritten values assigned in markup.
This works well for the majority of attributes and properties that take a single value. However, the class attribute can be assigned a list of space-separated class names. In JavaScript, the list of class names can be appended to, prepended to, overwritten, deleted, or otherwise arbitrarily modified. Sometimes, multiple JavaScript functions will modify the class attribute of the same element in different ways.