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.
while using bgcolor=”#ffffff” instead of bgcolor=#ffffff results is greater browser compatibility and differentiates it nicely from other attribute-value pairs.
Quoting attribute values is also required in XHTML. Your pages will fail the W3C validation if you leave the values hanking around with the quotes.