I will not assume that you know Ruby, but I will assume you know the basics of programming. For example, you should know what a variable is, or a loop.
It is possible to write Watir tests with almost no Ruby knowledge, but the more you know Ruby, the easier it will be for you. If you grow to like Ruby, there is plenty of literature that will help you learn more.
I will not assume that you know a lot about HTML, CSS, JavaScript, AJAX, DOM and stuff like that, but you will have to know the basics. For example, you will have to understand what a HTML tag is, what tag attributes are and stuff like that.
I have not made up the prerequisites. I have just listed everything I knew when I started using Watir.
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