问题
More and more, began to notice that the links in the source code on Web sites begin with two slashes. For example:
<a href="//example.com/1.png">Image</a>
Why do it?
回答1:
It's a protocol-relative URL (typically HTTP or HTTPS). So if I'm on http://example.org and I link (or include an image, script, etc.) to //example.com/1.png, it goes to http://example.com/1.png. If I'm on https://example.org, it goes to https://example.com/1.png.
This lets you easily avoid mixed content security errors.
来源：https://stackoverflow.com/questions/7273573/links-start-with-two-slashes