The premise is that you want to make something that competes with Safari, e.g. you want to provide a graphics API to third party developers.
Doing that in Safari doesn't work twice. First, if Safari doesn't already expose whatever GPU primitives you need to do things efficiently, you can't get them. And second, your API implementation (which is likely to be large) would then have to be distributed by anyone using it instead of being part of the browser they use for all sites, creating a >100MB dependency for a <50kB web page.
Doing that in Safari doesn't work twice. First, if Safari doesn't already expose whatever GPU primitives you need to do things efficiently, you can't get them. And second, your API implementation (which is likely to be large) would then have to be distributed by anyone using it instead of being part of the browser they use for all sites, creating a >100MB dependency for a <50kB web page.