What library does Capybara use as CSS selector?

Question

For my acceptance tests I use Steak and Capybara (+ Envjs). When trying to find an element like this find("#ticker").find("li:eq(0)") it tells me Unable to find 'li:eq(0)', but find("#ticker").find("li:eq(1)") return the correct first li element in the list. I am used to that eq selector starts with 0 (jQuery). Why does eq selector in Capybara starts with 1? Bug or feature?

Update

It seems that it is some unofficial selector which works like nth-child. What does Capybara (/Envjs) use under the hood for selectors? It doesn't seem to be jQuery as it would start with eq(0) as mentioned above.

Answer 1

Capybara uses Nokogiri under the hood.