HTML Checking for Large Sites
Rocket Validator integrates the W3C Validator HTML checker into an automated web crawler.
Attribute “placeholder” is only allowed when the input type is “email”, “number”, “password”, “search”, “tel”, “text”, or “url”.
placeholder attribute is used on
<input> elements as text that appears in the form control when it has no value set. Only inputs of type
url accept this attribute.
For example, an input of type
hidden doesn’t accept a
placeholder attribute as it makes no sense to specify text to appear when the input is empty, if that input is hidden.