When a RadPane with not configured fixed size is locked its size is not calculated properly (and the size of all its sibling free-sized panes).The issue can be reproduced with the following code in Firefox (both in horizontally or vertically oriented Splitter):

    <telerik:RadSplitter ID="TestSplitter" runat="server" Width="100%" Height="100%"
        Orientation="Horizontal">
        <telerik:RadPane ID="RadPane1" runat="server"
            Locked="true">
        </telerik:RadPane>
        <telerik:RadPane ID="RadPane2" runat="server">
        </telerik:RadPane>
        <telerik:RadPane ID="RadPane3" runat="server">
        </telerik:RadPane>
    </telerik:RadSplitter>

These overrides can be used as a temporary workaround:

        Telerik.Web.UI.SplitterPaneBase.prototype.get_height = function () {
            return this._height === "" ? 0 : parseInt(this._height);
        }
        Telerik.Web.UI.SplitterPaneBase.prototype.get_width = function () {
            return this._width === "" ? 0 : parseInt(this._width);
        }

The HTML5 related changes in the RadSplitter have specific selectors that influence the content inside.

When RadSplitter is declared in a page with no Doctype the RadSlidingPane stops responding in IE and the layout looks broken when docking/undocking.

When the content of a RadPane is set through its ContentURL property, the RadPane cannot be scrolled in order to how its full content in IPAD.

The issue could be reproduced here: http://demos.telerik.com/aspnet-ajax/splitter/examples/externalcontentloading/defaultcs.aspx