In the lesson, the text says that adjectives ending in "u" become "üe" in the feminine (at least in writing). Later in the lesson, it says that before 1990 the alternative "üe"  was the acceptable spelling and is still considered acceptable.

So if "üe" is an alternative but acceptable spelling then what is considered the modern acceptable spelling? Is this a typo in the lesson? 

Full quotes included below from the lesson for context:

Note that adjectives ending in -u (and NOT derived from verbs) become -üe in the written form, but are pronounced in exactly the same way as the masculine form. 

Note that before the 1990s, the alternative -uë was the accepted spelling, and is still deemed acceptable now.