I have a question where I don't have any idea for a solution at the moment.
We have built a responsive landing page using the method with two closing div's (</div> </div> at the beginning).
But when we now want to add a form using the normal drag and drop form didn't work (because it's with absolute positioning) and destroys the layout. When we add it using the HTML iframe code the form will not be pre-filled.
Steffan, in the landing page editor, edit your Page Elements (like your form) with either posOverride, posOverrideRight, or posOverrideFull to float the element for positioning. See below:
FYI: posOverride floats left, posOverrideRight floats right, and posOverrideFull floats to the left with 100% width.
*Props to Edward Unthank from Yesler for identifying this technique. I'd recommend contacting him if you need help with responsive Marketo LPs.
PS. BTW, depending on what responsive CSS you're using (e.g. Twitter Bootstrap), you can also adjust where your Marketo elements will be placed by editing your template. In the proper div Class (or article/section), you'll want to include <?php echo $mContext['bodyElements']; ?> for your landing page elements.