For users with a Chinese locale, it's better to use GBK as the fallback encoding. It uses the same decoder as gb18030, but it uses a more conservative encoder which is likely more compatible with servers.
Chrome and Firefox already have (this) behavior (like this). (Unfortunately there are still some differences in the overall text encoding story. Mozilla would be up for standardizing those.)
https://github.com/whatwg/html/pull/4714 changed this in the HTML Standard.
Test page: https://hsivonen.com/test/moz/sniff-zh-hans.htm
Says "windows-1252" on my machine. But apparently, if you change your system language to Chinese and reboot, it will say "gb18030" (not "gbk").