Displays all emails in offline HTML format

I use IMAP to sync to Gmail. When offline, all my emails are displayed in plain text format, images and layouts are not displayed properly. Is there a way so that the images and email formats can all be downloaded and displayed correctly when I’m offline?

No. When you are offline, you cannot download anything, independent of which programme you use.

I think you mean to preload and cache contents while you are online for later offline usage. - Here too, the answer is no as this would pose an immense security risk (unsupervised download of potentially harmful content).