In an extension's template file, when you have these two lines:

<!-- INCLUDEJS test.js -->

<!-- INCLUDECSS test.css -->

The JavaScript file is included, but the CSS file is not. This is weird behaviour and undesirable for extension developers.

The cause of it is {$STYLESHEETS} already being parsed before the extension template file is handled. (INCLUDECSS currently only works in one specific template event)

A way this could be fixed is to not directly output template files when they are read, but first read them and afterwards parse them all at once.