The RadEditor do not have automatic logic that strips the onload attribute. Also, testing the same scenario on my end, I am unable to reproduce it. You can find attached a sample page which I have been using to follow the described situation.
Are there any further details on the matter that could be relevant e.g., additional JS customizations or server-side filtering? The version with which I have been testing is 2014.2.724, let me know if you refer to an older one.
On a side note, with the upcoming Q3 2014 version, the RadEditor control will have a built-in filter that will strip all DOM attribute events. You should be aware that upon upgrading to this filter, the StripDomEventAttributes
filter should be disabled. There will be more inforamiotn on the matter in the Content Filters article
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