Nielsen Norman Group: Placeholders in Form Fields Are Harmful
This is a good article to point at when someone is suggesting that placeholders serve as the sole label for input fields. The lack of labels is pervasive on mobile apps and sites. While not having labels conserves space and can make for a cleaner form, it can also make the form much less usable–especially when it comes to accessibility.
There are ways combine the clean look of a form without labels with the usability of clearly displayed labels. The float label pattern is one example. The Polymer framework includes this pattern with the floatingLabel attribute in Paper Elements, although it is turned off by default. It would be wonderful if more mobile interfaces moved away from hiding labels in favor of patterns like the floating label.