For general web development questions that are not specifically related to CSS HTML Validator. This includes (but is not limited to) general HTML, CSS, Accessibility, JavaScript, and SEO questions.
-prefix-free lets you use only unprefixed CSS properties everywhere. It works behind the scenes, adding the current browser’s prefix to any CSS code, only when it’s needed.
A similar solution that doesn't require JavaScript is here (paste in your CSS and get the "prefixed" results back): http://expressprefixr.herokuapp.com/