Hello Simon,
The HTML5 is still not fully supported by most of the browsers, and each of them has different problems. Could you explain what exact markup you are using and how it behaves in different browsers. If the problem is inside the RadTextBox we will fix it, but if it is browser problem and can be replicated using regular ASP TextBox or <input> element, there is no something that we can do about it.
Regards,
Vasil
Telerik
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