Browser & Platform Compatibility
Fifteen years ago, design considerations were considerably simpler than they are now. Most people worked on a PC, running Internet Explorer (or at a stretch, Firefox) and most desktops or laptops worked off roughly the same resolution. The parameters in which a website needed to be adjusted for were relatively slim.
Fast-forward to present day and it couldn’t be any more different.
You have multiple operating system platforms. There’s a plethora of browsers, each of which have different versions that do/don’t support modern web technologies. There’s now a huge array of mobile devices that are all touch based.
All these things have to be catered for when designing and building a site, and a lot of the time supporting one, breaks another. This is all just part of the challenge!