Do Not Use Protocol Relative URL in Email Campaigns
The Protocol relative URL is awesome! If you do use these types of links, be sure to NOT use them in your email campaigns. Depending on the email client that is being used, the link will be broken. Protocol relative links received in Mac Mail app will turn
If you can use either http or https you really should be using and forcing https. It makes the web a better place and can help prevent things like GitHub’s man on the side attack happening to you.