Relative references are not suitable for emails because the absolute path will be of the URL where the email is opened, not where image is stored.
<div width="100">
To ensure compatabilty accross all emails clients Campaign Cleaner will convert the attribute with into a style width also.
<div style="width:100px" width="100">
Some attributes after the conversion will be removed, like bgcolor
, some we will leave both in place, because it can affect the rendering layout of your newsletter.