The savePage API allows you to save a tab as MHTML.

MHTML is a standard format supported by most browsers. It encapsulates in a single file a page and all its resources (CSS files, images..).

Note that for security reasons a MHTML file can only be loaded from the file system and that it can only be loaded in the main frame.

Manifest

The infobars API is currently experimental, so you must declare the "experimental" permission to use it.