We have seen validation errors (we used W3C validator at https://validator.w3.org/) that are related to colspan being set to number that is less than the number of col elements defined in colgroup. Example:
<col style="display:none;" />
We have seen these errors in your RadGrid demo source too (http://demos.telerik.com/aspnet-ajax/grid/examples/overview/defaultcs.aspx)
When can we expect this issue to be resolved?