A de facto standard is one that is actually in use. It typically implies limits and constraints that prevent the adoption of any "better" alternative. For instance:
While none of these is mandated by any standards organization, they are nonetheless very difficult to argue with or work around. In general one has no choice but to adopt them or do a great deal of work to accomodate them.
- the de facto standard for wiki is mediawiki and its wikistyle format
- the de facto standard for name precedents on the Internet is GFDL corpus namespace
- the de facto standard for web browsers is Internet Explorer
- the de facto standard base software for boot images is Microsoft Windows
While none of these is mandated by any standards organization, they are nonetheless very difficult to argue with or work around. In general one has no choice but to adopt them or do a great deal of work to accomodate them.