Is there any normative source that is explicit about what's "correct"?

The various SQL standards, I presume. But they may leave it as "implementation defined". Anyway, from the Sybase 15 manuals:

Use n to specify the number of bytes of storage for char and varchar datatypes. For unichar, use n to specify the number of Unicode characters (the amount of storage allocated is 2 bytes per character). For nchar and nvarchar, n is the number of characters (the amount of storage allocated is n times the number of bytes per characer for the server’s current default character set).
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Use n to specify the maximum length in characters for the variable-length datatypes, varchar(n), univarchar(n), and nvarchar(n). Data in variable-length columns is stripped of trailing blanks; storage size is the actual length of the data entered. Data in variable-length variables and parameters retains all trailing blanks, but is not padded to the defined length. Character literals are treated as variable-length datatypes.
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Fixed-length columns tend to take more storage space than variable-length columns, but are accessed somewhat faster. Table 1-17 summarizes the storage requirements of the different character datatypes:

Table 1-17: Character datatypes

Datatype        Stores                        Bytes of storage

char(n)         Character                     n
unichar(n)      Unicode character             n*@@unicharsize (@@unicharsize equals 2)
nchar(n)        National character            n*@@ncharsize
varchar(n)      Character varying             Actual number of characters entered
univarchar(n)   Unicode character varying     Actual number of characters * @@unicharsize
nvarchar(n)     National character varying    Actual number of characters * @@ncharsize

I'm a bit surprised Sybase doesn't need to store the length of a stored varchar. The chapter about storage sizes of datatypes repeats the information above.

MySql defines their storages sizes here. For varchar, the storage size is defined as L + 1 bytes if column values require 0 – 255 bytes, L + 2 bytes if values may require more than 255 bytes, where L represents the actual length in bytes of a given string value..