The doctype has never been useful for detecting browser features. You needed, and still need to, do that with user agent sniffing or with Javascript. All the do

The doctype has never been useful for detecting browser features. You needed, and still need to, do that with user agent sniffing or with Javascript. All the doctype has ever been useful for is for controlling the browser rendering mode, i.e. standards compliant mode or quirks mode. HTML5 defines much more precisely how pages should be rendered, including how to render broken HTML pages, thereby obsoleting a thing like quirks mode.