There are several issues you should consider before, during, and after you use styles in your web documents and document collections. The first, overarching issue is whether to use them at all. Frankly, few of the style effects are unique; most can be achieved, albeit less easily and with much less consistency, via the physical and content-based style tags like <i> and <em> and the various tag attributes like color and background.
We think the CSS2 standard is a winner, not only over just JavaScript-based standards, but mostly for the convenience and effectiveness of all your mark-up documents, including HTML, XHTML, and any and most other XML-compliant ones. The majority of browsers in use today support CSS2 styles. The benefits are clear. So, why wouldn't you use styles?