ERROR := ε
WS := U+0009 | U+000A | U+000D | U+0020

pseudoattr_name := ( ANYTHING - ( '=' | WS | EOF ) )*
pseudoattr_value_double_quoted := ( ANYTHING - ( '<' | '"' | EOF ) | '<' ERROR )*
pseudoattr_value_single_quoted := ( ANYTHING - ( '<' | ''' | EOF ) | '<' ERROR )*

psuedoattrs := ( WS* pseudoattr )* WS* ( '=' ERROR | EOF )
pseudoattr :=
    psuedoattr_name
    (   EOF ERROR
    |   ( '=' | WS ) WS*
            ( ( ANYTHING - '=' ) ERROR
            | '=' WS*
                ( '"' pseudoattr_value_double_quoted ( EOF ERROR | '"' )
                | ''' pseudoattr_value_single_quoted ( EOF ERROR | ''' )
                | ( ANYTHING - ( '"' | ''' ) ERROR )
                )
            )
    )

Match the data attribute against the pseudoattrs production. It will always match unless this spec is buggy (which it almost certainly is).

For each pseudoattr production that was matched:
  If no ERROR terminal was matched in this production:
    Let name be the pseudoattr_name production that was matched. (There will always be one, unless this spec is buggy.)
    Let value be the pseudoattr_value_double_quoted production that was matched or, if it wasn't, then pseudoattr_value_single_quoted. (There will always be one of these, unless this spec is buggy.)
    If name is not already in the psuedo-attributes array:
      Append (name, value) to the psuedo-attributes array