CSS: best and bad practices in Wordpress theme development

One of my daily tasks as a developer is to modify the default styles which come along with a new Wordpress theme chosen by our clients. I have to say that this task is made harder and harder by some obtuse practices followed by many theme developers when it comes to CSS. I don't want to be rude because I love your work, but you should consider the fact that your styles may be changed by another developer or user, so you should code your CSS accordingly. Here are some of these bad practices that make me feel frustrated.