Okay...thanks.
I attributed it to jetpack because the warning only appeared when I had this plugin active.
Maybe you know the answer this. I looked up the documentation on warning and it says the one I'm getting is because the site doesn't have a valide doctype, but it definitely does. (https://github.com/jquery/jquery-migrate/blob/master/warnings.md#jqmigrate-jquery-is-not-compatible-with-quirks-mode)