Users need feedback from websites. Buttons, links, and other interactive elements should respond to elementary user input. All web designers probably try to account for user feedback, especially in controls like buttons and links, but a lot of websites have strange ways of letting the user know what he can or can't do. There are some de facto standards from the software visual interface world that apply to web design, as well as a few guidelines that make pliant response more effective.
Pliant response is feedback that indicates the manipulability or effect of a control.
A lot of websites have menus made of buttons whose appearances change when the user moves the cursor over the button area. I'm going to focus on that, and closely related pliant response mechanisms. No pliant response is generally bad. If buttons on a website don't light up, or otherwise change in appearance, then there is often very little to distinguish them from other graphic elements. One common confusing mechanism involves rollovers that result in a description either appearing elsewhere on the page, or replacing the original button image.