The rounded corners of RadEditor consist of table cells with a inside, which is in fact a symbol. That is why when you are using certain browser settings such as font-size a cell dislocation occurs. On the other hand, we cannot remove the  , because having empty cells on a page triggers another two problems - empty elements is not xhtml compliant and empty cells cause a number of other issues, as the empty-cells property is not implemented correctly in any browser.
What I can suggest here is to keep using the fix you have invented, but, please have in mind that the usage of the zoom and font settings in the browsers is really rarely used.
the Telerik team
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