I want to display a counter written out. So using the h1 counter ...

"Article 1" should be "Article One"

I have ...

content: "Article " counter(h1);

... but I can't figure out which property to use to get the number to spell out.

Currently there is no counter style that uses words, although you can do this by hand using the symbols counter style:

content: "Article " counter(h1, symbols('One','Two','Three','Four'));