I'm a web developer. I most often come across this critical situation when the website layout fits with the design when run on Windows 7 or 8 and Mobile Operating Systems (Android, iOs, Windows 8).
But when the website is visited by old operating systems like Windows XP and Windows 1998 the whole layout collapses very badly. And specially when visited on old operating systems with Internet Explorer.
If we take into account the old operating systems (and/or IE on them) it creates havoc for us. Sometimes, even the well supported front-end frameworks like Twitter Bootstrap give up there and won't support the older browsers.
When clients or management request to make the website compatible with all the OS out there including Win XP, how to address this?