Shebang definition

Hello,

I wonder where the '#!' shebang is defined. I thought it is part of
SUSv3, but it doesn't seem to be required (see 'environ, execl, execv,
execle, execve, execlp, execvp - execute a file' and 'sh - shell,
the standard command language interpreter'.
In the latter one finds 'Furthermore, on systems that support executable
scripts (the "#!" construct)[...]', this hints that it is common but not
required [cf. the section 'Historically there have been two ways that
implementations can exec shell scripts.' in 'environ, execl ...'].

Shouldn't therefore the LSB define/require the support of the shebang?

Tobias