Web development and the "data" URL, where is it defined?

Quick pointer for today: in modern web development, it is more and more common to specify small image files as inline data. This can be done using the so-called data URLs, which can look like the following:

<img src="data:image/png;base64,A83e1jdi..." />

But the question is, where is this data URL format actually specified? As with many things related to the Internet, the source is one of those RFC documents. In this case, it's RFC 2397 from August, 1998. Yes, almost 15 year ago!