They&#x27;re already enabled in Firefox, and they&#x27;ll also be supported in the next version of Safari.<p>I&#x27;m going to use these to allow changing the appearance of my app, which has a Windows 95-style UI. 95 has many customisable values that determine how the UI looks (fonts, font sizes, font styles, foreground and background colours, UI element sizes, etc.) that it stores in certain registry keys and that the user can change from the Display control panel applet. In my app, I&#x27;ll be using CSS variables in place of registry keys.