Tach,

Steht das so in der Spezifikation, dass sie nicht gemeinsam verwendet werden dürfen?

Kann ich mir nicht vorstellen, auch wenn ich es (so kurz vor Feierabend) nicht nachgeschlagen habe.

Warum Doku lesen, wenn man stattdessen spekulieren kann? ;-)

There are three possible modes that can be selected using these attributes. If the async attribute is present, then the script will be executed asynchronously, as soon as it is available. If the async attribute is not present but the defer attribute is present, then the script is executed when the page has finished parsing. If neither attribute is present, then the script is fetched and executed immediately, before the user agent continues parsing the page.

[…]

The defer attribute may be specified even if the async attribute is specified, to cause legacy Web browsers that only support defer (and not async) to fall back to the defer behavior instead of the synchronous blocking behavior that is the default.

mfg
Woodfighter