The values of attributes can contain text, hexadecimal color codes or numbers. In case of JavaScript event handlers, the values consist of small JavaScript code. A sincere advice for good and clean HTML coding is to ALWAYS put quotes around attribute values. It is a good habit, will save you many headaches and avoid errors especially when attribute values contain spaces.

Thus, using BGCOLOR="#FFFFFF" instead of BGCOLOR=#FFFFFF results is greater browser compatibility and differentiates it nicely from the next attribute-value pair.