There are several reasons why UA detection is relevant where feature detection fails. The first is that you can serve prebuilt js binaries that have only the relevant logic for the requesting browser; strictly smaller js files for every user since you aren't serving irrelevant js.
Some things aren't feature detectable, like actual weird bugs in behavior that need browser specific workarounds.
In this case it looks like MS has the domain listed for their fallback mode, which doesn't just change the UA but actually changes the browser behavior (IE9 in IE8 mode has no canvas support and will fail the feature detection for canvas appropriately).
Some things aren't feature detectable, like actual weird bugs in behavior that need browser specific workarounds.
In this case it looks like MS has the domain listed for their fallback mode, which doesn't just change the UA but actually changes the browser behavior (IE9 in IE8 mode has no canvas support and will fail the feature detection for canvas appropriately).