I've written a JavaScript-related question, and I tagged it with cross-browser, but it was (for my unfortunate surprise) marked as portability, which is its synonym master. And it wasn't just me who found that weird: user 200_success also demonstrated to be unsure on what portability tag meant in my question, and despite both might appear to be interchangeable, they actually aren't, as we can note in their tag wiki descriptions:
Portable code can be run with little to no modification in multiple environments. Portable applications can be run from, for example, a USB drive without modifying a computer's environment.
Cross-browser refers to the ability of a website, web application, HTML construct or client-side script to function in environments that provide its required features and to bow out or degrade gracefully when features are absent or lacking.
We can also note disambiguation evidences in Simon's comment and Joe's answer.
What do you think about desynonymizing them? How could we handle that change?