I've used those before, but I feel that having something that works within the spec, independent of any particular implementation's extensions is much more valu

I've used those before, but I feel that having something that works within the spec, independent of any particular implementation's extensions is much more valuable for longevity. Sure, you can use $A if using $B server, or use $C when using $D language, or use $E when using $F framework, or use $G when using $H reverse-proxy, or use $I when using $J CDN, or use $K when using $L hosted cloud service ... Or you can just use what's in the specification which works on all of the above, but won't work on clients with Javascript disabled. It's a "choose your poison" scenario.