PPK nails it as usual:
Back-enders work with only one environment, their server. They have the option of changing their environment; for instance by installing new software or upgrading the hardware.
Front-enders work with an unknown number of radically different environments, ranging from state-of-the-art desktop computer to three-year-old, crappy phones with limited memory and processor time. They cannot change these environments because they have no power over their users’ browsers. Still, their code should work in all of them.
I shared similar thoughts in my post A Fundamental Disconnect.