Elements of a website’s User Interface need to be obvious. Said another way, don’t confuse users with ambiguous design choices.

Take a look at the following checkbox, a common form element:

Ambiguous checkbox design which contains a checkmark. Is it ticked?

Now ask yourself if the box is ticked. Well, there is a checkmark in the box so maybe it has already been ticked. But no, this is what the ticked checkbox looks like:

The checkbox in its ticked state. The checkbox looks darker when ticked

It’s not obvious, is it? There’s room for improvement here. Make your elements obvious.