An attribute must not be specified in a start tag using a name token group or notation value without a preceding attribute name and VI or without surrounding literal delimiters, if SHORTTAG is NO.

#0137   Markup Error

ISO 8879, 7.9.1.2

An attribute must not be specified in a start tag using a name token group or notation value without a preceding attribute name and VI or without surrounding literal delimiters, if SHORTTAG is NO.