First of all, I would like to mention, that the RadSplitter is actually a table, not a DIV element - the element you refer to is just the wrapper element.
One of the main principles that we follow, when we develop our controls is to get consistent behavior under all supported browsers. As you probably know, that is not an easy task. There are various reasons why we have decided to keep the size of the elements in the RadSplitter in pixels. For example, the Scrolling functionality or the MinWidth/MinHeight and the MaxWidth/MaxHeight functionality of the RadPanes.
I agree that for simple scenarios, using css styles to set the Width of the RadSplitter in percents is a better approach, but unfortunately, this approach would prevents us from providing all the functionality that the controls currently offers.
All the best,
the Telerik team
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