A WordPress plugin or theme should never have to output inline styles, especially not in the frontend. Inline styles will prevent some doctypes from validating and they are actually a factor to slow down your page loads.
If your plugin outputs a piece of code that needs styling, add an ID to the output and output the style using a
style tag in the
<head>. Next to that you should add an option to remove the style altogether, and tell users how to add the CSS to their own theme’s stylesheet.