HTMLEditor disable automatic calculation of the width

I use the HTML editor. I try to edit an image with specified width in percentages. If I switch to design mode, the width is not anymore specified in percentages. It has a fixed value in pixels.

Is there a way to disable the automatic calculation of the width in design mode?

Here is an example

I type <img src="http://www.asp.net/ajaxLibrary/AjaxControlToolkitSampleSite/ASPNET/ajax_logo.png" width="15%">

than switch to designmode and back again i have

<img width="220" src="http://www.asp.net/ajaxLibrary/AjaxControlToolkitSampleSite/ASPNET/ajax_logo.png" />