In this particular case, the RadTextBox has valid HTML output. Since it is strict by the HTML specification, if there is any problem, it must be in the generator that you use. So I would suggest you to report this issue activepdf.
You could to hard-code the same HTML as you quoted here, into your page, and remove the classes that you think can be a problem and see if it will be exported.
Generally we can not guarantee compatibility with another third party software. And we can not change our rendering because of problem with the WebGrabber, because it is currently valid and it shows correct in the browsers.
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