Just downloaded a template from the Marketo templates page. I know a little HTML in order to manipulate certain features, but I'm having an error validating my recent change. Essentially, all I'm looking to do is copy and past code already in the template in order to duplicate a particular section. I was able to do that, and sent a test email that appeared to come in fine, but the template won't validate in the Design Studio. I'm not sure exactly what to change in order to validate, but it seems like a simple fix if someone could lend a helping hand! Thanks in advance for any follow up.
Uf what you have done was just to duplicate a section, then it's likely that you have duplicate IDs.
search the code in you duplicate section for "mktoEditable" word. it will come into a string such as class="mktoEditable", which it self will be included in an HTML tag such as <div> or <td>. In the tag, there will also be an id="something" attribute. Just add anything at the end of the "something" so that you get id="something1".
That should do the trick.