HTML and CSS alone would not be adequate for the kind of behaviour you are looking for - 'updating their site'. At the very least, you need to learn how to customise a CMS. A well known CMS that I would recommend is WordPress. However, there are others in other languages/platforms. But, since you are talking of a CMS, I thought I should point you towards the most widely used one. Thus, you need to invest your time in learning how that CMS works and what you need to do to customise it for your clients. You might also need to learn something about databases and MySQL would also be another good recommendation.