It ' d be weird if he still weren ' t TwoFace yet . <| sep |> It ' s not theory , it ' s practice . I ' ve been doing web development for the better part of 15 years . I still have fond memories of battling every day to get both basic and complex designs to work across IE and Firefox . Chrome wasn ' t even a thing then , much less Safari . And no one gave a shit about Opera then either . At least that hasn ' t changed . We had to use CSS bugs in IE , and much more rarely for other browsers , to address certain layout issues . That was as device specific as you needed to get . And those issues are basically all gone today . IE has removed conditional comments for a reason . They aren ' t needed anymore . In terms of js , in the early days there were a lot of issues between browser features but again feature detection was used . Old xml is a great example of that . So these days there are so many different kinds of devices it ' s impossible to target them all individually . The one exception to this is if you are writing a web app specifically for one platform like iOS mobile Safari . But in this case , since you ' re writing for a single platform there ' s no need to do device detection anyway . Moot point . But yeah , I ' ll happily stand by my point of view . If you think you need device detection , you ' re approaching it from the wrong end . Stop and rethink about what you ' re doing