Should Cross browser testing be explicitly mentioned in the scope of a project?

Question

I do freelance web development and front end dev is not my strongest point. This question came to me in my recent fixed bid project.

Nowadays we use Jquery and Bootstrap and these take care of lots of cross browser stuff. Still, there are lots of errors in different browsers. It requires testing tools and a whole lot of other things to worry about.

Should it be explicitly mentioned that the resulting site should be tested for cross browser compatibility in a contract? Or

Does it come by default in the scope of the project?

How can we estimate hours spent in cross browser compatibility?

Edit: Some great answers here. But how should a scenario be handled when no browser discussions happened at all at the time of signing contract?