El atributo placeholder no es una etiqueta, no sustituye a label

En Placeholder Attribute Is Not A Label!:

Just so we’re all clear on this, the HTML5 placeholder attribute in a text input is not a replacement for the label element. Period. The placeholder should only be used as a brief example of the text to be entered. 

Besides inconsistent support for screen readers, using a placeholder as an input label can create usability problems and issues for those with cognitive impairments. For example, how does one review the information entered if the placeholder is now gone? 

The placeholder should be used like a title attribute (tooltip); it provides only supplementary information. If the information is required for the user (such as a strict text format) then this should be conveyed in the main content of the page, not in an attribute.