here's the IDL file from webkit's TextArea, you can see that
placeholder is supposed to exist.
interface HTMLTextAreaElement : HTMLElement {
attribute [ConvertNullToNullString] DOMString defaultValue;
readonly attribute HTMLFormElement form;
readonly attribute ValidityState validity;
attribute [Reflect] DOMString accessKey;
attribute long cols;
attribute [Reflect] boolean disabled;
attribute [Reflect] boolean autofocus;
attribute long maxLength setter raises(DOMException);
attribute [ConvertNullToNullString] DOMString name;
attribute [Reflect] DOMString placeholder;
attribute [Reflect] boolean readOnly;
attribute [Reflect] boolean required;
attribute long rows;
readonly attribute DOMString type;
attribute [ConvertNullToNullString] DOMString value;
readonly attribute unsigned long textLength;