Consider the following examples:
Protocol Relative CSS URL
<link href="//example.com/wp-content/themes/twentytwelve/style.css" />
More appropriately called a "network-path reference" or "scheme-relative path", a protocol-relative URL helps to avoid common scenarios such as:
The idea of using protocol-relative URLs was popularized by Paul Irish and has become common as sites switch between serving HTTP and HTTPS assets.
This plugin will standardize all properly enqueued theme files to use protocol-relative paths, even those added from third-party plugins.