HTML::Tokenizer - where is it

I’m looking at the source code of ActionView and I came across this line

tokenizer = HTML

I want to see the documentation for HTML and the tokenizer method but couldn’t find it anywhere. Obviously I’m a new to Ruby and Rails world. Could someone let me know where I could find the documentation for the Tokenizer method.

If I need to use the the “strip_tags” method in a non rails what do I need to include to have this method in my program?