What does // mean in an <a> tag

Question

I'm writing a web crawler and I'm testing it out by starting at Wikipedia. However, I noticed that many of wikipedia's links are prefaced with //, so the link from wikipedia.org to en.wikipedia.org is a link to //en.wikipedia.org. What exactly does this // mean in practice? Does it say "use whatever scheme you were using before and then redirect to this url?" or does it mean something entirely different?

Answer 1

The link will use protocol (http or https) same as page which contain that link. For example if https://stackoverflow.com/ contain <a href="//en.wikipedia.org"></a> it will directed to https://en.wikipedia.org

Answer 2

It maintains the protocol that is being used for the webpage. HTTP/HTTPS.

It's particulaly useful for external scripts and css tags, in which you don't know in which protocol your site will be working on.

That's why on Google libraries (https://developers.google.com/speed/libraries/devguide#jquery) you have like this:

<script src="//ajax.googleapis.com/ajax/libs/jquery/1.10.2/jquery.min.js"></script>

Just while writing this I found a duplicate: Two forward slashes in a url/src/href attribute

Take a look at it.