Most probably the problem comes from a known limitation of Internet Explorer - all style tags after the first 30 style tags on an HTML page are not applied. This IE specific most often concerns exactly the RadToolTip control because its CSS is alphabetically one of the last declared on the page. This problem can occur in case you are using a theme with a lot of CSS (when you register the theme, automatically all the CSS is declared) or you have declared a lot of RadControls on the page. If you are using a theme you can test my assumption by removing it and test whether the RadToolTip CSS will load correctly.
If this is the case I suggest to either use a RadStyleSheetManager which will combine all the CSS on the page or reduce the CSS.
A more detailed blog about this is available below:
the Telerik team
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