Input Placeholder 介绍
This plugin extends the jQuery() class with .placehold() method, which is
effective for text inputs and treats the default value those inputs have in
HTML markup as a placehold value.
When user focuses an input field, its placehold value has to be cleared out,
but it returns back, if user leaves the field having put nothing in there.
Additionally, my method can add a specified class to active and inactive
fields separately, enabling you to style and access placeholded and active
inputs.
Please, remember to validate the form on submit, additionally checking fields
for containing a placeholder, because placeholder strings are obviously
treated as normal input values by UAs. If a user submits such form, it might
even appear valid on the server side, filling your DB with bogus data