When deleting all content (e.g. Select All [CTRL+A] + Delete [Del]) in the document, it would be convenient for the user to preserve all formatting (local span formatting + local paragraph formatting + style) applied on the last paragraph. 

This will ensure that the user will continue edit the document with last current font, font family, etc.

If this operation is applied on an empty document, though, the styling of the single paragraph should be reset to the defaults for the document.