Browser prefixes are used in order to force developers to wait for standardization, but still enable them to have an early access. Those features are experimental, the implementation might be buggy or based on different drafts and hence look different in different browsers. It is logical to prefix such dangerous features, which might break a homepage (at least once they are standardized). Keep in mind: the standard might be different from the draft on which the implementation is based!
Further Reading: MDN