(although maybe that only works on mobile browsers)

The link:before ... content to expose an attribute is a CSS feature, not a mobile-browser feature.

The @media shown is what restricts my solution to browsers like used on mobile devices, but using no @media or a different @media definition can make it work on desktops as well.

While debugging the CSS, I actually didn't use the @media, so I could see it on my desktop browser (since that's where I was editing the CSS anyway) to make sure it was working, then I added in the @media and made sure it stopped showing on desktop but did still show on mobile.