Users with a variety of visual disabilities, such as colour blindness, focusing problems, are most affected by the wrong combination of background and text colours. As well, the practice of adding large blocks of white on white text to increase search engine scores is invisible to the average viewer, but a screen reader user will be bombarded with this information. If colour alone is used to convey information, people who cannot differentiate between certain colours and users with devices that have non-colour or non-visual displays will not receive the information.
When defining colors for text, ensure that the text is readable against the background and that links are easily recognized. When colour is used properly, it can actually make reading easier and aid in the user's comprehension, however, don't rely on colur alone to convey meaning.
Font size is often used as a more flexible alternative to headers. One practice, called Drop Caps, is to use a larger font size for the first letter of a word than for the rest of the word. An HTML aware screen reader will read this as two separate terms; for example, when Drop Caps is applied to the word "The", a screen reader will say "tee hee" because the word is broken by the FONT open and close tags in the underlying HTML. It is for this reason, as well as general HTML style, that Cascading Style Sheets (CSS) are recommended to change sizes of text.