Hi Richard:
You can do this in Preview mode of a landing page. The code of the actual landing page is iFramed into the preview pane. Chrome's "Inspect Element" is a moderately easy way to go about this.
If you're looking for unaltered, pure HTML for a landing page... you can't get that until you see the approved page.
I personally just name the URL something random at first, approve it to see the code, and then my final personal "I approve this page" is by changing the URL to something SEO-sensical. It's much easier check out the HTML and CSS and JS with the approved page. Especially because the Marketo editor doesn't call external CSS/JS except for on the approved page.
Best,
Edward UnthankMarketing Operations Specialist
Yesler