If you have the opportunity, and since you have to upgrade ASAP anyway since 7.12 has been withdrawn: upgrade to 7.13 My colleagues pointed out that in 7.13 there is again a login.html replacing the index.html. So likely you're problem will be solved as well.
In your username and password fields you can write what you want for your placeholder text. I just confirmed that the placeholder text works in 7.12.
Maybe try cleaning your cache or deployment directory when testing it? Let me know if it still doesnt work.