Reduce the amount of tags that use the style attribute, replacing it with external CSS definitions. Is this supposed to be for performance or are they just sug

Reduce the amount of tags that use the style attribute, replacing it with external CSS definitions. Is this supposed to be for performance or are they just suggesting "good practices"? If it's performance, I'd like to see some evidence that style attributes slow down the DOM. I don't know of any, and it would be significant.