ACM requires author-year-style citations but I have several references such as web pages that have neither an identifiable author and don't state their year of publication. How should I cite such documents?
The ACM's guidelines give the following examples of how to format a bibliography entry for a website:
Website year can be found at the bottom of the website page or by viewing page properties/source to see when the page was last modified.
 Harry Thornburg. 2001. Introduction to Bayesian Statistics. (March 2001). Retrieved March 2, 2005 from http://ccrma.stanford.edu/~jos/bayes/bayes.html
 ACM. Association for Computing Machinery: Advancing Computing as a Science & Profession. Retrieved from http://www.acm.org/.
 Wikipedia. 2017. Wikipedia: the Free Encyclopedia. Retrieved from https://www.wikipedia.org/.
I'm not sure why  doesn't have a date. Although that page only covers numerical citations, I think it's reasonable to infer that author-year citations for  and  would be (Thornburg, 2001) and (Wikipedia, 2017).
APA says to use "n.d." for "no date"; I'd be tempted to either use "undated" or use the date you accessed the page (but make it clear that's what you're doing, in that case). I've also seen papers that cite websites using footnotes and only put fully citable things like papers, books and theses in the bibliography.