It is the webapp developer who ultimately decides whether or not there is mixed content, not the browser. If you don't mix content in your webapp, an attacker w

It is the webapp developer who ultimately decides whether or not there is mixed content, not the browser. If you don't mix content in your webapp, an attacker who controls the network shouldn't be able to change your content (not even to inject references to new untrusted HTTPS or plain HTTP servers), or that of trusted service providers. The browser needs to implement SSL securely, but even users with a browser with no mixed-content warning benefit from there being no mixed-content. The mixed content warning helps to warn the developer of the site of the problem, and let users of browsers that support it know that they are not fully protected.