The ::-moz-placeholder pseudo-element represents any form element displaying placeholder text. This allows web developers and theme designers to customize the appearance of placeholder text, which is a light grey color by default. This may not work well if you've changed the background color of your form fields to be a similar color, for example, so you can use this pseudo-class to change the placeholder text color.

[Webplatform](http://docs.webplatform.org/wiki/css/selectors/pseudo-classes/:-ms-input-placeholder "Webplatform")
[Mozilla docs](https://developer.mozilla.org/en-US/docs/Web/CSS/::-moz-placeholder "Mozilla")
