It means that id
should look like this:
<form action='' id='contact_form'>
E. g. without any whitespaces in value of id
attribute. Also, it should be unique.
You haven't specified whether you are validating your document against HTML4 or HTML5. And there are some differences in specs for id
in HTML4 and HTML5 - you can read about both here:
HTML 5:
HTML 4.0: