This smacks of the bad old days of the web, where UA checking was de rigour (usually to lock sites to IE). And then, as now, the page usually functioned fine u

This smacks of the bad old days of the web, where UA checking was de rigour (usually to lock sites to IE). And then, as now, the page usually functioned fine under alternatives with a masked UA. "Warning: You browser is unsupported. You may experience issues. Continue?" is the only defensible roadblock.