Properly, the --foo stuff is a Custom Property.  You can <i>use</i> its value as a variable, with var(), but it&#x27;s not the only use custom properties have.  That&#x27;s why the spec&#x27;s full name is &quot;CSS Custom Properties for Cascading Variables&quot;.  But that&#x27;s a horrible mouthful, so &quot;CSS Variables&quot; is what it&#x27;s usually called.  (Plus that&#x27;s the shortname in the url.)<p>(There&#x27;s also a large focus, in v1 here, on the variables use-case, because that&#x27;s what I needed to sell it to the group at the time.  The CSS Houdini Task Force is now expanding Custom Properties <i>properly</i>, so they can actually be used for random JS things without a ton of hackery.)