For me in the early 2000's, it meant having entirely separate codebases to support IE and Netacape. It also meant that building a web application with async data loading (XML from a REST service) was all done by hand. My favorite technical achievement at the time was a front end that allowed managing hierarchies of accounts. It was essentially a single page application and allowed dragging and dropping of things. One of the hardest parts of that to get right was duplicating and shadowing the elements you were dragging so you saw the actual stuff being moved, not just the default drag icon.
Oh, and lots of Visual Basic on the back end.