You should have a good understanding of UI design. You should know to line up buttons, keep similar elements consistent from page to page, keep font styles and colors consistent, etc. You should know that primary actions typically use a darker or bolder style, and secondary actions typically use a lighter style. For example, when there is a Submit and a Cancel button, the Submit button should be in the primary style, and the Cancel button would be in a secondary or lighter style such as grey or pastel.
Your CSS should be clean and organized. You should not be defining the same font styles in many different classes and elements. You should not use inline styles.
You should be familiar with the bootstrap framework and the grid system. You should also be able to use divs and CSS to build responsive layouts. You should understand how to use background images so that they stretch proportionally and reduce properly on smaller screens.
Projects will be ecommerce websites and other various web applications.