It's the website developer's responsibility to switch it over. Using them in the first place is saying, "I want to use cutting edge features that may change an

It's the website developer's responsibility to switch it over. Using them in the first place is saying, "I want to use cutting edge features that may change and break at any time." Being removed and replaced with the standard version is just a special case. But it doesn't hurt to use multiple versions, so most people just specify it for each browser and the standard. The browser will ignore the ones it doesn't understand. For example, this works: div { -webkit-border-radius: ...; -moz-border-radius: ...; border-radius: ...; }