The "problem" with protocol-relative URLs, is that it's possible to include HTTP content if the parent was HTTP. Since HTTPs is preferred for both security and

The "problem" with protocol-relative URLs, is that it's possible to include HTTP content if the parent was HTTP. Since HTTPs is preferred for both security and privacy, we should give as little options as possible to use the insecure HTTP protocol and force HTTPs everywhere. As you mention though, in dev-environments it's a convenient hack to use plain old HTTP. However, in production, preferring HTTPs would be considered the way to go.