The Document of a content attribute is the Document of the attribute’s element. Authors are encouraged to make use of conformance checkers (also known as validators) to catch common mistakes. The attribute value can remain unquoted if it doesn’t contain space characters or any of » ‘ ` = < or >. Otherwise, it has to be quoted using either single or double quotes. There is no implied relationship between document conformance requirements and implementation conformance requirements. However, a PDF viewer application that launches separate from the user agent (as opposed to using the same interface) is not a plugin by this definition.