We are familiar with this issue. Currently there is no workaround for that but we are working on and it will be fixed soon. Here is some additional information:
In general, RadFormDecorator decorates the controls on the client. This leads to undesired effect similar to flickering when the page is initially loaded - the users were able to see the un-decorated controls first. To fix this problem, we added an additional style on the page that was placed in the head section. This fixed the problem, but raised another one when RadStyleSheetManager (that combines all the styles) was used.
Currently we are researching different options to fix that problem and once we have a solution, we will provide a hotfix right away. If however, at this point it is mandatory for you to have 100% valid XHTML output, our suggestion is not to use RadFormDecorator for now.
All the best,
the Telerik team
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