If the webpage is offline, how'd you serve the form and process submissions from people? When you approve a landing page, Marketo makes it live and accessible via the public internet, and given that people have the URL to it, or they are able to get it from the search engines, they could access your LP (I don't think you could make an approved LP live for a closed group of people, or for people on your intranet). However, if you want to have your team test this before approving the LP, you could use the Preview URL functionality (snapshot below). You don't need to make your LP live (aka approve it) for generating the preview URL.

Until the LP isn't approved (or doesn't have any approved drafts), the LP URL would re-direct to your fallback address defined in the Admin > Landing Page.